Ford code p1131

Ford code p1131 DEFAULT
Trouble CodeFault LocationProbable Cause
PLack Of HO2S Switch - Sensor Indicates Lean (Ford, Lincoln, Mazda, Mercury)
Swirl Control Valve Control Solenoid Valve (Infiniti, Nissan)
Injection Quantity Adjust (Hyundai)

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What Does Code P Mean?

The upstream heated O2 sensor provides the Powertrain Control Module or PCM with the information needed on how rich or how lean the engine is running. It is located in the exhaust system before the first catalytic converter. This is done in order to see just exactly how rich or how lean the engine is running without the catalytic converters affecting the makeup of the gases in the exhaust. The upstream heated O2 sensors, or just HO2S accomplishes this by providing a 0 – 1volt signal to the PCM, with .5 volts as the crossover point. If the voltage is above .5, the engine is said to be running rich. If below .5 volts, the engine is said to be running lean. The voltage should be switching back and forth between rich and lean so as to average out the indicated air/fuel mixture, or ratio.

Code P is a manufacturer’s specific code and as such lets you know that diagnosing this condition may be different between manufacturer’s vehicles. In these instances, P will set when the voltage being returned by the sensor is no longer switching above and below approximately .5 volts

What are the common causes of code P?

  • Engine vacuum leak &#; most common
  • Restricted fuel injectors – next most likely, especially if poor quality fuel has been used/encountered
  • Fuel pressure too low – possible
  • MAF sensor dirty/failed – possible
  • Failed Heated O2 Sensor – possible
  • Failed PCM – unlikely

What are the symptoms of code P?

  • Malfunction Indicator Light “ON”
  • Possibly poor fuel economy

How do you troubleshoot code P?

First, take a look and see if there are any technical service bulletins (TSB) for your particular vehicle. There may be an update, or known fix put out by the manufacturer that can save you from wasting time and money.

Next, see if there are any other diagnostic fault codes. Diagnose current faults first, in the order in which they are stored. Misdiagnosis occurs when this code is diagnosed when it is a stored code, especially while other codes are active. If these codes are present, diagnose them before attempting to diagnose the P

If the P is the only active fault code present, and there are no updates/TSBs for your particular vehicle, then the next step is to locate the upstream Heated O2 Sensor for bank 1 on your particular vehicle. Typically, it will be found close to, if not directly screwed into the exhaust manifold that cylinder 1 exhaust gases flow out of. Once located, visually inspect the connectors and wiring. Look for the obvious damage to the connector. Note any chafing, rubbing, bare wires, burn spots or melted plastic. If there is damage to the wiring to the point of bare wires touching the frame or to a ground (bare metal spot on the vehicle), replace the sensor.

If you replaced the sensor and have a scan tool, clear the diagnostic trouble codes from memory, and see if this code returns. Because the PCM must run the engine both rich and lean to check the HO2S, it may take several days for this code to return. Your scan tool may be able to give you “Pending Codes”, in which case you will know if and when the P is about to return. If the P does not return, then the connections/wiring issues were most likely your problem.

If the P code does return, further testing will be required. We will need to test the Mass Airflow Sensor or MAF sensor, as it is a major airflow input to the PCM. The easiest way of testing the MAF sensor is by monitoring the voltage signal from the sensor to the PCM on a scan tool.  If a scan tool is not available, then test the signal coming from the MAF sensor with a digital volt ohm meter (DVOM). With the sensor connected, the red voltmeter lead should be attached to the MAF sensor signal wire and the black voltmeter lead connected to ground. Start the engine and monitor the MAF sensor input. As engine RPM is increased, the MAF sensor signal should increase. Check the manufacturer’s specifications, as there may be a chart informing you of what the voltage should be at a given RPM. If it fails this test, replace the MAF sensor and retest.

If all tests have passed so far, and you continue to get a P code, monitor the HO2S. If it continually indicates that the engine is running lean (voltage continually below .5 volts), locate any and all possibilities that could cause a lean running engine. These include but not limited to:

  • Intake or exhaust leaks
  • Fuel system, including fuel pressure/fuel pressure regulator.
  • Fuel pressure sensor
  • Fuel injectors
  • O2 sensor after the catalytic converter

If the HO2S indicates that the engine is running normal or even rich, a failed PCM could not be ruled out until all other issues have been eliminated. If unsure, seek assistance from a trained automotive diagnostician. And remember, PCMs must be programmed, or calibrated to the vehicle in order to be installed correctly.

Codes Related to P

P &#; Upstream Heated O2 Sensor or Fuel System Not Switching Indicating Lean Bank 2 &#;  Same as P but for bank 2.

P and P – System Too Lean Bank 1 and Bank 2 &#;  Same diagnostic indications and steps as P and P, but their requirements to set are different.

Other Manufacturer Specific Definitions for P

Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 1 (Ford)
HO2S Circuit Low Variance Bank 1 Sensor 2 (GM)
HO2S21 Inner Resistance Too Large (Volkswagen)
Brake system vacuum sensor – signal malfunction (Acura)
Heated oxygen sensor (HO2S) 1, Bank 1 - heater resistance too high (Audi)
'A' Camshaft Position Plausibility (BMW)
HO2S Circuit Low Variance Bank 1 Sensor 2 (Buick)
HO2S Circuit Low Variance Bank 1 Sensor 2 (Cadillac)
HO2S Circuit Low Variance Bank 1 Sensor 2 (Chevrolet)
Glow Plug Control Module Internal Fault-Voltage Supply Concern (Chrysler)
Injection Quantity Adjust (Hyundai)
Intake manifold air control solenoid – malfunction (Infiniti)
Heated oxygen sensor (H02S), upstream, bank 1 – mixture too lean (Land Rover)
Heated oxygen sensor (H02S) 1, bank 1 -notswitching, fuel trim (FT) weak mixture (Lincoln)
Heated oxygen sensor (H02S) 1, bank 1 – lean mixture indicated (Mazda)
Heated oxygen sensor (H02S) 1 – signal/ electrical fault (Mercedes-Benz)
Heated oxygen sensor (H02S) 1, bank 1 not switching, fuel trim (FT) weak mixture (Mercury)
Oxygen Sensor Behind Catalytic Converter 2 Ageing (Mini)
Intake manifold air control solenoid – malfunction (Nissan)
Heated oxygen sensor (H02S) 1, bank 1 – malfunction (Saab)
Heated oxygen sensor (H02S) 1 -short circuit (Subaru)
Heated oxygen sensor (H02S) 1, bank 1 – output voltage high (Suzuki)
Heated oxygen sensor (H02S) 1, bank 1 – signal malfunction (Volvo)

BAT Team Discussions for P

  • Taurus SE - P///errors
    Vin code digit #8 is "U". I did screw up on the codesP=fuel trim system lean bank 2 and P=fuel trim system lean bank 1. The P and P were correct codes. Our initial thought is to put in a fuel pump but hate to and find that was not the issue, would think a fuel pump would also p
  • Codes or Mazda Protege
    I have only seen a P Lean, be cured by cleaning the MAF. I have no records of your codes. Can you tell us the year of your car just in case there may be a difference in Mazda codes through the years
  • Stop and Go traffic??
    Problem is getting worse. Car is starting to buck now at certain timesand getting code P What to check or do first? Thanks! RB
  • Ford Windstar Lean Bank 1 and 2, and misfire
    To bad lots of info got lost in the site crash, I posted a while back a really long post about intake manifold runner controls, I think Kev was referring to those, look for broken tabs at manifold runner control, you will see the rod for the runners just hanging and the tabs missing or lying on the
  • Taurus Wagon GL: Repair "H" hose, now won't start
    , miles. I pushed down on the wiring harness when reattaching the (now) somewhat shorter "H" hose. Thought this could be a problem, so I then ran the "H" hose UNDER the harness (which seemed better). Any chance I could've broken a wire in the harness? I didn't move it very much at all, but

P is a diagnostic trouble code that applies to vehicles manufactured by Ford, including the Ranger. Trouble codes that start with P1XXX were reserved for manufacturers. This code occurs when the upstream O2 sensor on bank 1 fails a diagnostic test twice in a row.

P Ford Ranger

P Quick Facts

  • DTC P is usually caused by a bad oxygen sensor, fuel injector, or vacuum leak. But, there can be other causes as well.
  • This code does not normally cause any drivability issues.
  • P is a Ford code (Mercury, Ford, Lincoln)
  • Affects bank 1.  When it&#;s bank 2, the code is P

Definition and How to Find Bank 1

P Diagnosis Ford Ranger


Ford chose P to indicate

Upstream– The upstream oxygen sensor is the sensor before your Ranger&#;s catalytic converter. It’s usually bolted to the exhaust manifold or right after it.

Bank 1– Bank 1 is the part of the engine with the first cylinder. If your Ford has a four cylinder engine, this doesn&#;t apply, as there is only one bank. Bank 1 is easy enough to find. Here’s a short article on how to find which side of the engine has bank 1. It’ll open in another tab.

Once you find it come back here and we’ll try and figure out what’s causing P in your Ranger.

Ford Ranger:  P Symptoms

Typically, there are not going to be any symptoms associated with P (other than the check engine light).  You might (big might here) smell the faint scent of fuel.

Common P Causes

  • Fuel Injectors can cause P
  • Bad Fuel Injectors can cause P, but it’s more likely to be a bad O2 sensor.
  • Bad O2 Sensor (Bank 1 Sensor 1): See above if you need help locating it.
  • Vacuum Leak
  • Fuel Pressure Issue: The fuel pressure could be too high or low. 
  • Bad Mass Airflow Sensor
  • PCM (The vehicle’s computer)

Ford Ranger: P Diagnosis

P Diagnosis

There are some things that you can do at home to detect what is causing P and repair it. Here they are in order (somewhat) of likelihood/ease to check.

Bad O2 Sensor Wiring

The upstream oxygen sensor on your Ranger is easy enough to find. Once you know where bank 1 is, just follow the exhaust from the manifold until you find the oxygen sensor.

Inspect the wiring harness to see if it has been damaged in any way. Look at the contacts where they plug into the sensor itself. Due to the location of an oxygen sensor on hot exhaust components, the harness fails more than just about any wiring harness on a vehicle.

Related:  Ford Ranger Bad O2 Sensor Diagnosis

Bad Oxygen Sensor

P is triggered because the oxygen sensor failed a test. Specifically, a test is designed to make sure that the oxygen sensor has the proper output voltage and response rate. It must fail this test twice in order for the check engine light to come on. 

With that in mind, knowing that the oxygen sensor has failed this test, it’s most likely the reason your Ranger has P

Vacuum Leak

The system could have a vacuum leak. Determining where a vacuum leak is can be easy. Here&#;s a great video on exactly how to do that.

Fuel System

The fuel system itself can cause P

The fuel pressure could be too low. This would be caused by a clogged fuel filter, weak fuel pump, or bad fuel pressure regulator. But, If the fuel pressure is too low, you’d likely get P (same code, just the other side of the engine) as well.

With the above in mind, if the fuel system is to blame, it’s likely an issue with a fuel injector.

A clogged fuel injector can lean the mix out on bank 1, without affecting bank 2.

Wrap Up

As you can see, P can be caused by a lot of issues. It’ll more than likely be a bad oxygen sensor, fuel injector, or vacuum leak that is causing it.  Outside of that, it can be pretty challenging for a DIY guy to track it down.

Good luck diagnosing your vehicle. If there’s anything you’d like to add, please leave  a comment.


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The P is an OBD2 Diagnostic Trouble Code that may seem harmless but will definitely lead to component damages and poor fuel economy.

This code is associated with the oxygen sensor and indicates whether the engine is running on a rich or lean fuel mixture.

It is one of the most commonly displayed Ford OBD 2 codes and prevalent in vehicles such as; Ford Ranger, Ford Explorer, Ford F, Ford Escort, and even Ford Mustang. In this article, we will dive into its meaning, diagnosis, and fixes.


P Ford Code Definition – What Does It Mean?

General Code Definition

P trouble code on Ford vehicles is defined as “Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 1”.

However, the p is a manufacturer-specific trouble code, which means other brands will have different definitions.

Meaning Of P

The Powertrain Control Module (PCM) uses the upstream heated oxygen sensor (HO2S) to check if the engine runs on a lean or rich fuel mixture. The upstream HO2S is able to identify the amount of oxygen coming out in the exhaust gases.

From this, the Powertrain Control Module can analyze whether the engine is running on a rich fuel mixture (too much fuel with not enough air) or lean fuel mixture (too much air with not enough fuel). It then uses this information to compensate for any imbalances of the right mixture by injecting more or less fuel into the engine.

The HO2S does its job by providing voltage signals. The signals range from 0 volts to 1 volt, with as the crossover point. The engine is said to be running rich if the reading is above , while a reading below indicates a lean condition. For Ford vehicles, the P code will be displayed if the system isn’t switching between the volt ranges as much as it should.

Symptoms Of P Code In Ford

  1. The Check Engine Light (CEL)/Service Engine Soon Warning Light will turn On.
  2. May experience poor fuel economy.
  3. The vehicle may accelerate slowly.
  4. The engine may stall with occasional misfires.
  5. May experience a rise in exhaust gas emissions.
  6. Car may experience difficulties in starting.

Causes Of P Ford Code

  1. Engine vacuum leak after the MAF (Mass Air Flow) sensor.
  2. Restricted fuel filter, fuel injector, or air filter.
  3. Malfunctioning or broken HO2S.
  4. Malfunctioning or broken MAF sensor.
  5. Low fuel pressure.
  6. Fuel system malfunctioning.

How To Diagnose And Fix The Code


  1. Before diagnosing the p code, make sure to use the OBD scan tool to ensure no other codes are presently stored. If there are any, diagnose them first in the order in which they are stored. The reason for this is to prevent any misdiagnosis.
  2. Search online for any available Technical Service Bulletins (TSB) for your specific car and model. These are manufacturer fixes that are put out when a definitive solution has been found for that particular trouble code. Proceed to the diagnosis steps below if no TSB exists for your car.

Here is a list of tools that you will need for this diagnosis:

  1. OBD2 scan tool.
  2. Wrench, pliers, and spanners.
  3. Screwdrivers.
OBD2 scanner is a good tool to diagnose P

Diagnosing And Fixing

Step 1:

Connect the OBD scanner to the OBD port on your vehicle. Use the scan tool to check for any other stored codes. It will also provide comprehensive information on the problem to help you solve it quickly.

Follow any of the recommended guidelines. If none, proceed to step 2.

Step 2:

Locate the upstream heated oxygen sensor, usually screwed onto the exhaust manifold where exhaust gases flow out. Inspect the wiring and connection around the area. Check for burnt, corroded, or broken connections.

Replace the upstream heated oxygen sensor if any of the wires are bare or grounded.

Step 3:

Clear the p code and rescan to figure out if step 2 fixed the issue. After that, check for pending codes. It may take a few days for pending codes to disappear or reappear. Test the Mass Airflow (MAF) Sensor if the code returns.

You can test the MAF sensor using the scan tool to check the voltage signal from the MAF sensor to the PCM. Start the engine and monitor the sensor&#;s input. Your car manual and specifications sheet will have information on what the normal values should be.

Replace and rerun the above diagnosis to see whether the problem was with the MAF sensor.

Step 4:

If the above steps haven&#;t worked yet, check for any vacuum leaks, loose connections, and any fault in the fuel supply. You will need the wrench, spanner, and screwdrivers to do this. Your car manual will help you locate the specific locations. Tighten the spark plugs connecting wires, replace any malfunctioning hoses in the engine air-intake. Tighten and correctly place any loose components such as in the fuel assembly, the fuel injector, fuel filter, and air filter.

After Fixing

Clear the p code test the vehicle to see whether the code is returned. Rescan by using the scan tool and take note of any other codes that may be stored too. If none, then hurrah, you just fixed the issue.

All these steps will help you pinpoint the malfunction. In case the problem still persists, you will need to take your vehicle to a mechanic for further inspection. He/she may test for a failed PCM or other related components.

Tips To Avoid P Code

  1. Take your vehicle for servicing as regularly as possible. Car maintenance should be a priority so that you won’t have to fix problems when they arise but rather deal with them before they do.
  2. Always check for engine leaks.
  3. Keep an eye on the fuel assembly, always checking for fuel injection pressure and filters.
  4. You should replace corroded or burnt wirings as soon as possible.

Other Codes Related To The P Ford Code

  1. P – Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 2
  2. P &#; Lack of Heated Oxygen sensor switches Bank 1 sensor 2 indicates lean
  3. P – Lack of Heated Oxygen sensor switches Bank 2 sensor 2 indicates lean

Final Thoughts

Always aim at fixing OBD2 diagnostics trouble codes as soon as you can, regardless of the drivability of the vehicle.

Note that the p code may not affect your vehicle’s functioning to the point of breakdown. You can drive for a few weeks or months with a faulty or broken Mass Airflow sensor. However, over time, you will notice poor gas mileage, fuel economy, and the vehicle will begin to stall frequently. If left unchecked, the p related issues can cause serious damage to other parts of the vehicle.

Simply put, it is not recommended to drive a vehicle with the p code.

Here are some articles about other Ford codes that you may interested in:


The OBD II code P is quite common on most Ford Vehicles, including on Ford F, Ford Ranger, Ford Taurus, Ford Explorer, Ford Expedition, Ford Mustang, and Ford Escort.

Read this article to learn about P Ford code

Table of Contents

Introduction to P on Ford Vehicles

The error code P is displayed by getting feedback from the Lambda (or the oxygen) sensor. The Lambda sensor is responsible for detecting the amount of oxygen in the exhaust gases. Based on that, the PCM (Powertrain Control Module) manipulates whether the engine is running on a lean or rich fuel mixture.

A lean running condition of the engine happens when the engine runs on less fuel or too much air, ultimately resulting in the error code P on Ford vehicles.

An engine running on a lean mixture, may lead to sluggish acceleration and reduced fuel economy. Thus it becomes crucial to understand the P on Ford Vehicles and ways to resolve this error code.

Read more: Ford vs. W: Which engine should I choose?

Code P Ford Definition and Meaning

The Ford error code P is displayed by getting feedback from the the oxygen sensor.

In simple terms, the OBD II code P, when displayed on your Ford Vehicle, indicates the PCM detected a malfunction. By estimating the O2 amount, the PCM assesses whether the engine is running on a rich or lean fuel mixture. A lean mixture can give rise to various problems and thus displays the code P

Whereas, an optimized fuel mixture is preferred because of the following-

  • Better Fuel Economy
  • Better Acceleration
  • Reduced tailpipe emissions
  • Prolonged engine life 

In contrast, an engine running on a lean mixture can give rise to many issues that can appear combinedly or separately. That is discussed in the coming sections.

Some of The Other OBD II Codes That Are Related to P on Ford Vehicles are-

Symptoms of P on Ford

The most visible symptoms of the P code is-

Moreover, there are some other symptoms for the P code on Ford. But, the chances of encountering them are minimal-

  • Engine Stalling
  • Engine Knocking 
  • A rise in Exhaust gas Emissions 
  • Engine frequently misfires

Causes of P on Ford

The causes of P Ford error code.

There can be various causes for the error code P

1. Fuel System Malfunctioning

Issues in the Injectors, fuel filter, pump, etc., can result in a low quantity of fuel reaching the combustion chamber. The components listed below collectively or individually can cause Fuel System Malfunctioning- 

  • Clogged Fuel filter: This may create a barrier to fuel passing the filter during the filtering process, thus creating a lean mixture.
  • Defective Fuel Pump:A lean mixture can be formed due to the fuel pump&#;s inability to supply the fuel with the required optimal pressure.
  • Faulty Fuel Injectors: The injectors are responsible for spraying the fuel into the combustion chamber.  If they start to leak or become dirty, it may result in an interrupted fuel injection into the cylinder. The injectors with low pressure can result in the formation of a lean mixture. 

2. Oxygen Sensor Inaccuracy

At times, it is possible that the sensor responsible for measuring the O2 in the cylinder malfunctions. Thus, reporting more oxygen in the mixture than the actual quantity. The amount of oxygen is proportional to the air quantity supplied. As a result, the PCM may display a lean mixture. Thus displaying the P code.

3. Air Mass Flow Sensor Inaccuracy

The Air Mass Flow sensor estimates the amount of air that&#;s entering the cylinder. A faulty Air mass flow sensor may inaccurately assess the quantity of incoming air. Thus, as a result, the P code may be displayed on the Ford vehicle.

4. Computer Malfunction

Though the chances of malfunctioning of an on-board computer are very low, however, a faulty on-board diagnostics system can inaccurately display the P code.

5. Air Leaks

Innumerable hoses and gaskets are employed in an engine. These components are not built to last forever. Hence, it is possible due to incessant exposure to heat and pressure; any of these components gives rise to air leaks. Thus, air leaks can find its way into the engine, assisting in the formation of a lean mixture.

How Serious is POBD II Code on Ford

Though the issues causing the P code may not impact your vehicle&#;s functioning.  However, if ignored, these can cause severe damage to other vital components in the long term.

A regular check-up or servicing might help to resolve the problems responsible for OBD II code P Additionally, looking for the causes mentioned above can help fathom the issue much faster and easier.

Can You Drive with p?

It is not recommended to drive your Ford while the P code is being displayed. 

A petrol engine running on a lean mixture can cause problems like-

  • Reduced Performance
  • Engine Knocking (Inconsistent fuel combustion)
  • Damage to Piston

A Diesel engine running on a lean mixture  may not result in the same problems but still can cause-

  • Low Output from engine
  • Engine Stalling

However, the vehicle with the P code can cause sudden jerks with lagging acceleration. Moreover, it may reduce the efficiency of the vehicle while increasing emissions. 

Tools Needed to Fix any of these Issues-

How to Fix the Code P on Ford?

The first and foremost thing to be done when checking for the causes of the P code is to connect a car diagnostic scanner to the car&#;s OBD. Running a diagnostic scan can give comprehensive information on the malfunctioned system to help resolve the issue promptly. 

Moreover, if running an OBD diagnostics does not helps then, follow these steps to fix the code P

Step 1&#; Check and Replace any damaged engine air-intake hoses

Step 2&#; Tighten the Spark Plugs Connecting Wires

Step 3&#; Check and Resolve any issue with Distributor assembly, like rotor or cap

Step 4&#; Check any fault in the fuel supply

Step 5&#; Lastly, but importantly check the wiring and connection of the Oxygen Sensor.  

If the issue still goes undetected, then visiting a service center might help.

Tips to Avoid p in the Future

Here are a few Tips to Avoid OBD II Code P

  • Servicing your Ford F/ Ford Ranger/ Ford Taurus/ Ford Explorer/ Ford Expedition/ Ford Mustang/ Ford Escort regularly
  • Replacing worn off engine gaskets and hoses
  • Monitoring the fuel injection pressure from the injectors
  • Checking for any Engine air leaks

Frequently Asked Questions

Q) Why do lambda (oxygen) sensors fail?

  • Lambda sensor is continuously exposed to the high temperature of exhaust gases. Thus heat and vibration can cause damage to the wirings and connections. Moreover, the lambda sensor can be faulty due to corrosion as well. 

Q) What happens if my O2 sensor is damaged?

  • A damaged O2 sensor doesn&#;t cause any serious problems. You can even drive with a bad O2 sensor. However, it may hamper other engine components in the long term and can even affect the overall vehicle&#;s performance.

Q) My Ford is driving fine, but it shows code P?

  • This could be because of the malfunction of any of the components responsible for diagnostics, or your Ford is driving fine because the issue is still in its initial phase and can increase over time.

Read more : 5 Best Ford Scan Tools Review [Update]


P1131 ford code

Error Code P – Lack of Upstream Heated Oxygen Sensor Switch


Error Code P is described as Lack of Upstream Heated Oxygen Sensor Switch – Sensor Indicates Lean – Bank No. 1.


Error Code P refers to insufficient switching  of oxygen sensor bank 1 or the sensor 1 is lean. This trouble code indicates that the system is correcting the ‘rich’ for an overly ‘lean’ condition in engine’s air fuel ratio. The Heater Oxygen Sensor voltage is less than V. This is because the upstream O2 sensor (one closes to the engine), is not switching rich to lean and vice versa as often as it should.

If the diagnostic trouble code shows Error Code that starts with P1xxx, this means it’s a manufacturer specific code. Thus, you need to be specific in your search and indicate the make of your vehicle.

Oxygen SensorIn a nutshell, Error Code P on your Ford vehicle indicates a trouble in your O2 sensor. The O2 sensor’s job is to measure the amount of oxygen in the exhaust, and determine whether the air fuel ratio in the cylinder is too rich or too lean. An optimized air fuel ratio mixture means better fuel economy and fewer exhaust emission. This error code refers to lean condition, which means there is too much air in the air fuel ratio mixture.

Combustion engines, such as the one we have in our vehicle, runs by a mixture of air and fuel at around to 1— parts air to 1 part fuel. If the ratio goes lower than parts, this is called “rich” mixture. If it goes above parts, it is referred as “lean” mixture.

To define:

  • Rich Mixture = Too much fuel, not enough air
  • Lean Mixture = Too much air, not enough fuel

And to keep the engine running properly, the ECM (Engine Control Module) measures the oxygen content in the exhaust through O2 sensors and compensates any imbalances of the right mixture; by injecting more or less fuel.

Common Symptoms

  • Check Engine light is on
  • Engine misfire

Possible Causes

  • Vacuum leak after MAF
  • Rich fuel mixture
  • Restricted fuel filter, or anything that affects the fuel mixture
  • Restricted air filter
  • Bad PCV valve or PCV system hose
  • Low fuel pressure
  • Low engine coolant level
  • Leaking intake manifold gasket
  • Incorrect engine thermostat (stuck open or too cold)
  • Plugged injector

How to Diagnose

  1. Start with a vacuum test to determine whether there are any leaks in the system.
  2. Check the fuel injectors in the bank 1 cylinders.
  3. Determine when was the last time the fuel filters were changes, just in case you see it clogged enough to interfere the normal flow.
  4. Inspect the Exhaust Gas Recirculation known as the EGR valve to determine whether it is clogged or stuck open.
  5. On most vehicles, the engine idle is usually determined by the computer of the vehicle, but for some, you need to have it checked to make sure it is always on par with the recommended settings, especially if the vehicle just had a tune up.
  6. Lastly, the Oxygen sensor itself may be in bad condition, especially if it is cold. If you can’t find any other causes of the error code from your diagnosis, then you most probably have a defective O2 sensor.

You can also check the O2 sensor using a DVOM (Digital Volt Ohm Meter). Connect the device to the signal wire from the O2 sensor, warm the engine up to its operating temperature, and then disconnect the brake booster hose to induce a lean condition. The O2 sensor voltage should drop down to near zero.  Then, reconnect the host and spray some aerosol carb cleaner to the engine air intake; the O2 sensor voltage should spike to around 9/10 of volt from the rich condition. If the sensor responds as described, it is operational.

Then, using a thermometer, measure the thermostat opening temperature. If it doesn’t reach ° F or if the coolant level is low, you will get rich condition.

How to Repair

If the vehicle has a lot of miles on it, the easiest way to get rid of this error code is to replace the O2 sensor as part of its maintenance. Or replace at around , miles.

As said in the diagnosis above, you need to conduct a vacuum leak test first. Eliminate this possibility before proceeding to the air filter. When the code is still on, then you can also change your fuel filter.

Measure your thermostat opening temperature with a thermometer. If the engine is not reaching degrees F or if the coolant level is low, it will cause a rich condition and set the code you describe.

Replace faulty parts, then clear the code and see if the problem comes back.

Parting Tips

Do NOT buy and replace any parts of your vehicle until you have fully determined the actual cause of the problem through thorough testing. Last thing you want is to throw any functional parts. You should know that Error Codes don’t indicate specific component failures, rather, they say general system or circuit has gone abnormal. Thus, it is important to perform manual test to pin point the root cause of the problem.

For more guides and troubleshooting tips of different Error Codes, check out our website. For professional help, Contact Us.

Mercury Sable - Ford Taurus Duratec P1131 and P1151 easy and cheap fix

P FORD Code - Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 1

&#;What is P FORD code meaning?

The Heated Oxygen Sensor(HO2S) Monitor is an on-board strategy designed to monitor the HO2S sensors for a malfunction or deterioration that can affect emissions. Under specific conditions, the fuel control or upstream HO2S sensors are checked for proper output voltage and response rate (the time it takes to switch from lean to rich or rich to lean). Downstream HO2S sensors used for Catalyst Monitor are also monitored for proper output voltage. Input is required from the Engine Coolant Temperature (ECT) or Cylinder Head Temperature (CHT), Inlet Air Temperature (IAT), Mass Air Flow (MAF), Throttle Position (TP) and Crankshaft Position (CKP) sensors to activate the HO2S Monitor. The Fuel System Monitor and Misfire Detection Monitor must also have completed successfully before the HO2S Monitor is enabled.
• The HO2S sensor senses the oxygen content in the exhaust flow and outputs a voltage between zero and volt. Lean of stoichiometric (air/fuel ratio of approximately for gasoline engines), the HO2S will generate a voltage between zero and volt. Rich of stoichiometric, the HO2S will generate a voltage between and volt.
• The HO2S Monitor evaluates both the upstream (Fuel Control) and downstream (Catalyst Monitor) HO2S for proper function.
• Once the HO2S Monitor is enabled, the upstream HO2S signal voltage amplitude and response frequency are checked. Excessive voltage is determined by comparing the HO2S signal voltage to a maximum calibratable threshold voltage.
• A fixed frequency closed loop fuel control routine is executed and the upstream HO2S voltage amplitude and output response frequency are observed. A sample of the upstream HO2S signal is evaluated to determine if the sensor is capable of switching or has a slow response rate.
• An HO2S heater circuit fault is determined by turning the heater on and off and looking for a corresponding change in the OSM and by
measuring the current going through the heater circuit.
• The MIL is activated after a fault is detected on two consecutive OBD II drive cycles.

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