P1131 Ford Code: Diagnosing The Code

P1131 Ford Code: Diagnosing The Code

P1131 is amongst many of the OBD2 Diagnostic Trouble Codes that may be displayed on your Ford. While this may seem harmless at first, keep it hanging for long enough, leading to part damages and decreased fuel economy. The P1131 Ford code is linked to the oxygen sensor and signals the engine is running on a lean or rich fuel mixture.

It is one of the more commonly displayed codes for Ford vehicles and is prevalent in Ford Explorer, Ford Ranger, Ford Escort, Ford Mustang, and Ford F150. This article will discuss everything about the P1131 Ford code, from what it means to diagnosis and fixes.

P1131 Ford Code: What Does It Mean?

Error Code P1131 is displayed when sensor 1 or the oxygen sensor bank 1 does not switch enough to lean. This trouble code tells drivers that the system is fixing the overly “lean” situation with a “rich” one in the air-to-fuel ratio of the engine. The heated oxygen sensor voltage is below 0.45V since the upstream O2 sensor is not changing lean to rich and vice versa as frequently as it should.

Any Error Code that begins with P1xxx is a manufacturer-specific diagnostic trouble code. Therefore, you must be particular in your search and consider your vehicle’s make. In short, a P1131 Ford code indicates an issue with the O2 sensor in a Ford vehicle.

The job of the O2 sensor is to calculate the volume of oxygen in the exhaust and decide whether the fuel-to-air ratio in the cylinder is too lean or too rich. When the ratio is perfect, it leads to lesser exhaust emissions and improved fuel economy. An error like this indicates a lean condition, meaning there is an excess of air in the fuel-to-air mixture.

P1131 Ford Code

Combustion engines use a mixture of fuel and air at around 1 to 14.7 – 1 part fuel to 14.7 parts air. When the ratio dips under 14.7 parts, we term that a “rich” mixture. Conversely, when the mixture goes over 14.7 parts, that is referred to as a “lean” mixture.

Lean mixture: Too much air, too little fuel
Rich mixture: Too much fuel, too little air

Importance Of Optimising The Fuel-Air Ratio

To keep the engine functioning correctly, the Engine Control Module, or ECM, measures the exhaust’s oxygen content through O2 sensors and makes up for any imbalance in the blends by injecting less or more fuel.

In simpler terms, when a Ford vehicle displays the OBD II P1131 indicates a malfunction detected by the ECM. By calculating the amount of O2, the ECM checks whether the engine is operating on a lean or rich fuel mixture. A lean mixture can cause many problems, and so the vehicle displays the code as a warning sign.

An optimized fuel blend, on the other hand, will result in:

When an engine runs on a lean mixture, it can cause many issues that may or may not appear separately. That will be discussed in the upcoming segments.

Some other OBD II codes that can be associated with the P1131 Ford code include:

P1131 Ford Code: P1137

Lack of oxygen sensor switching on bank1 sensor. After the 3-way catalyst, the heated O2 sensor monitors the oxygen level in the exhaust for each bank. The air-to-fuel ratio must be maintained close to the ideal stoichiometric ratio for optimum catalyst operation. The HO2S2 output voltage shifts rapidly when the stoichiometric ratio is implemented.

P1131 Ford Code

The Powertrain Control Module, or PCM, regulates the fuel injection time to maintain the stoichiometric level of the air-to-fuel ratio. The HO2S2 generates a voltage of 0.1 to 0.9 V in response to the O2 found in the exhaust gas.

If the level of oxygen increases, the air-to-fuel mixture becomes lean. The PCM understands lean when the heated O2 sensor shows a voltage reading below 0.45 V. In contrast, if the O2 content in the exhaust gas reduces, the air-to-fuel mixture becomes rich. The HO2S2 displays a voltage over 0.45 V, and the PCM interprets that as rich. If the HO2S2 signal is under the voltage range for a prolonged period, the P1137 Ford code will be displayed.

P1131 Ford Code: P1151

Lack of upstream heated O2 sensor switch indicates lean in bank 2. Your vehicle’s heated oxygen sensor will signal you with this trouble code. As we mentioned before, this sensor tests the amount of oxygen to make sure the air-to-fuel mixture is accurate. A failed test implies there isn’t enough fuel blending with the air.

This can be the result of incorrect air intake. But, it can also be a sign that too much fuel is being injected. That being said, it can be a simple electrical issue with the powertrain control module or the heated O2 sensor in many cases. The PCM or HO2S sensor can be damaged, water lodged in the harness connector, or the wiring connectors might have gotten loose.

Any of the above-stated reasons could result in a wrong HO2S reading and make your vehicle display the trouble code P1151.

Moreover, too much fuel pressure, low fuel, or leaking fuel injectors could all be the reason behind this trouble code. Carefully inspect your vehicle’s fuel system to locate any contaminated or damaged components that would mess up the air and fuel balance.

A range of minor problems throughout the other systems may also cause this code. A leaky gasket in the EGR system or an air leak in the mass airflow sensor can be the reasons. An improperly seated engine oil dipstick can also cause this trouble code to be displayed.

The base engine can be suffering from an oil overfill, cylinder compression, or cam timing problems. Any exhaust leaks close to the heated oxygen sensor would result in a display of error code P1151.

P1131 Ford Code: P1157

Lack of HO2S22 Switches: Sensor Indicates Lean. The Air/Fuel Ratio (A/F) sensor 1 is located in the exhaust system and measures the oxygen content in the gas. It transmits output voltage to the ECM.

A heater is integrated into the A/F sensor. Once activated, it heats the sensor to speed and stabilize the measurement of O2 content by regulating the current flowing across the heater. The present amperage detects the air-to-fuel ratio while the ECM compares a fixed target air-to-fuel ratio to the detected air-to-fuel ratio and adjusts the fuel injection timing.

The air-to-fuel mixture ratio is lean if the A/F sensor voltage is low. The ECM uses feedback from the sensor to control the issue by injecting fuel into the mixture. If the A/F sensor voltage is high, know the air-to-fuel ratio is rich. Then, ECM uses A/F feedback control to repair the situation.

P1131 Ford Code: P0171 And P0174

System Too Lean Bank 1 and Bank 2. The diagnostics steps and indications are the same as P1151 and P1131, but the requirements are different.

P1131 Ford Code: P0171

The cylinder bank for lean code 1 is on the passenger’s right side of a V6 or V8 with RWD (rear-wheel drive).

P1131 Ford Code: P0174

Lean code cylinder bank 2 on the driver side of a transverse-mounted V6 with FWD (front-wheel drive). As a 4-cylinder does not come with bank 2, this does not apply to that engine.

These Ford codes can be seen when the PCM measures too much air-to-fuel mixture, deducing the mixture is too lean.

A dirty MAF sensor is the most common cause of this code. For this component to work properly, it has to be protected, which is supposed to be the job of the air filter. It has a tight seal that keeps the MAF safe.

When the seal is not secure enough, dirt gets stuck on the sensor wire and forms a coating that stops the mass airflow sensor from giving the correct reading. The sensor wire can also develop a coating of fuel vapors, which results in underreporting airflow.

Symptoms Of P1131 On Ford

How do you tell your car is showing P1131 Ford code? The most visible symptoms include:

There are a handful of other symptoms for the same issue, but the chances of those happening are somewhat slim-

Causes Of P1131 Ford Code

Why did your vehicle suddenly decide to give you such an issue? Well, these are the plausible causes of an error code P1131:

1. Fuel System Malfunction

Problems in the fuel filter, injector, pump, and so on can result in a lower volume of fuel getting to the combustion chamber. The parts listed below individually or collectively can make the fuel system malfunction:

2. Clogged Fuel Filter

A clog may stop the fuel from passing the filter in the filtration process, thus, making the mixture lean.

3. Defective Fuel Pump

A lean mixture could result from the fuel pump’s incapacity to deliver fuel with the needed optimal pressure.

4. Faulty Fuel Injectors

Injectors are tasked with spraying the fuel inside the combustion chamber. Dirty or leaky injectors may interrupt the smooth processing into the cylinder. Learn how to clean that here. Low pressure in the injectors can create a lean mixture, too.

5. Inaccuracy Of Oxygen Sensor

At times, the sensor responsible for measuring the oxygen in the cylinder might malfunction. Therefore, it reports more oxygen in the mix than the needed quantity. The volume of O2 is proportional to the quantity of air supplied, making the PCM display a lean mixture, and that’s when the P1131 Ford code pops up.

6. Inaccuracy Of Air Mass Flow Sensor

The air mass flow sensor measures the volume of air entering the cylinder. A fault in this sensor may make the entire system assess the incoming air’s quantity inaccurately. Thus, your Ford may display the P1131 trouble code.

7. Computer Malfunction

Though the possibilities of an onboard computer failing are scarce, a defective onboard diagnostics system might show the P1131 code incorrectly.

8. Air Leaks

An engine contains a plethora of hoses and gaskets. These components are not designed to endure indefinitely. As a result of constant exposure to pressure and heat, any of these components might cause air leaks. As a result, air leaks can enter the engine and contribute to a lean mixture.

How Serious Is A P1131OBD II Code On Ford?

Initially, the problems making the P1131 Ford code go off might not severely impact your car’s functioning. However, if ignored for an extended period, it can cause severe damage to other essential components. Regular checkups or servicing may help to resolve these issues early on. In addition, inspecting the causes mentioned can help you detect the issues faster and, thus, solve them quicker.

Can You Drive With P1131?

We do not recommend driving with a P1131 Ford code displayed on your vehicle. A petrol engine operating on a lean mixture could display issues such as:

  • Damage to piston
  • Engine knocking
  • Reduced performance

Similarly, a diesel engine operating on a lean mixture will show other problems, if not the same:

  • Engine stalling
  • Low engine output

However, a vehicle showing an error code P1131 can jerk suddenly along with lagging acceleration. It might decrease the efficiency of your vehicle while raising emissions.

How To Diagnose And Fix P1131 Ford Code


  • Before diagnosing the p1131 code, utilize the OBD scan tool to confirm that no other codes are currently saved. If any exist, diagnose them according to their saving order. This is done to avoid any possible misdiagnosis.
  • Look on the internet for any available TSB or Technical Service Bulletins for your car make and model. These manufacturer fixes have been implemented with a definitive solution for your specific trouble code. Proceed with the diagnosis if no TSB can be found for your vehicle.

Here is a list of tools that you will require to diagnose this trouble code:

Diagnosing And Fixing

If the only active error code is the P1131 and you cannot find any TSBs/updates for your specific vehicle, the next step is to find the upstream heated oxygen sensor for bank 1. It tends to vary between vehicles but generally can be found near, if not straight, screwed into the exhaust manifold that the exhaust gases of cylinder 1 flow out of.

Once found, visually check the wiring and connector. Look for noticeable signs of damage. Note any rubbing, burnt spots, chafing, bare wires, or melted plastic. Replace the sensor if the extent of damage to the wiring is bare wires coming in contact with the ground or frame.

If you change the sensor and own a scan tool, clear the diagnostic trouble codes. Check if the code returns as the PCM has to run the engine both lean and rick to check the heated O2 sensor, and it may take many days for the code to return.

Diagnostic trouble error message OBD scanner reader tool troubleshooting

A scan tool will provide “Pending Codes,” and you will know if and when there is a chance of the error code returning. If the code does not return, the problem is within the wiring/connections. Further testing will be needed if the error code does return.

Continue Testing

Next, you have to test the MAF sensor, or Mass Airflow Sensor, because it is a considerable airflow input for the PCM. The simplest way to check the MAF sensor is by observing the voltage signal transmitted by the sensor to the PCM via a scan tool. Under the circumstance that a scan tool is not available, test the signal with a digital volt-ohm meter (DVOM).

Connect the sensor and attach the red voltmeter to the MAF sensor signal wire. Connect the black voltmeter lead to the ground. Start your vehicle’s engine and inspect the MAF sensor input. The MAF sensor signal will increase (ideally).

Refer to the manufacturer’s specification because there might be a chart telling you the voltage applied at any RPM should be. Replace the MAF sensor if it fails the test and remembers to retest.

Still Getting Error Codes?

If you continue getting an error code despite passing all the tests, observe the H02S. If it keeps saying the engine is operating lean (voltage constantly under 0.5 V), find all possibilities that can result in an engine running on lean. These include but are not limited to:

If the heated O2 sensor says that the engine is operating regularly or rich, you can rule out a failed PCM until all other problems have been resolved. Seek professional help if unsure. Keep in mind that PCMs must be calibrated or programmed to the vehicle to be installed properly.

After Repairing

Clear the error code and test the vehicle to ensure it hasn’t returned. Use the scan tool to rescan and note any error code that might be stored. If the results come out clean, you just fixed the problem!

These steps will help you get to the bottom of the malfunction. If the problem persists, you have to bring your vehicle to a certified professional.

What Is The Cost Of Diagnosing The P1131 Ford code?

The cost of diagnosing a P1131 Ford code is an hour of labor. Auto repair shops’ labor rates and diagnosis time tend to differ by location, vehicle model and makes, and engine types. Most auto shops ask between $75 to $150 each hour.

Diagnostic trouble error message OBD scanner reader tool troubleshooting

Tips To Avoid P1131 In The Future

Given how complicated the P1131 Ford code can be to diagnose and fix, there is certainly no way you want to experience this problem again. Check out these tips to steer clear off OBD II Code P1131 in the future:

  • Has your vehicle been serviced as regularly as you can? Your priority should be car maintenance so that you don’t have to deal with the much worse problems that arise when your vehicle does not receive the care it deserves.
  • Replace the burnt or corroded wirings as soon as you notice them.
  • Monitor the fuel assembly, constantly checking for fuel injection filters and pressure.
  • Check for any possible engine air leaks.

Facts: What to Know About OBD2 Code P1131 for Ford Vehicles

  1. Code P1131 is a trouble code produced by the self-diagnostic system of Ford cars and indicates that the engine is working in a lean condition.
  2. This diagnostic code is commonly found in Ford vehicles such as the Explorer, Ranger, F150, Escort, and Mustang.
  3. Symptoms of engine code P1131 include a check engine light turning on, slow acceleration, failure to start, bad fuel economy, engine knocking, car stalling, and high gas emissions from the exhaust.
  4. Poor fuel economy and fewer exhaust emissions are the major consequences of error code P1131.
  5. Causes of code P1131 include a clogged fuel filter, malfunctioning fuel injectors or fuel pump, air leaking into the engine, faulty O2 sensor, airflow mass sensor error, or onboard computer error.
  6. The first step to fixing code P1131 is to make sure there are no other OBD codes and diagnose and solve them first if there are any.
  7. Fixes for code P1131 include checking for breaks in wiring and connectors of the O2 sensor, testing the airflow mass sensor, and checking for air leaks.
  8. Other fixes include replacing the fuel filters and injectors and thoroughly checking the fuel pressure sensor and fuel system.
  9. It is not recommended to drive with this error code for long distances as it can cause car stalling, poor performance, and damage to the piston.
  10. It is important to identify, locate, and solve the trouble code P1131 to prevent bigger issues and keep the engine healthy with better fuel economy and less dangerous tailpipe gas emissions.


Always try to fix OBD II diagnostics trouble codes quickly, regardless of whether you can drive your vehicle or not. What might not seem like a severe issue right now because your vehicle is still functional can turn into a dire problem in a moment. You will end up spending more to get that fixed than what you would have spent in repairing the error code early on.

Understand that the P1131 Ford code might not affect your Ford’s operation to the point of breakdown. Even with a broken or faulty mass airflow sensor, you should be able to drive around for at least a few weeks, maybe some months. However, poor gas mileage, frequent stalling, and decreased fuel economy will tell you that your vehicle is hurting over time.

If left unchecked, any error code, not just the P1131, can cause severe damages to other vital components of the vehicle. We highly suggest you not drive a vehicle with this error code displayed.

Frequently Asked Questions

Here are some popular FAQs:

Q. Why Do Oxygen Sensors Fail?

Fun fact: Oxygen sensors are also known as lambda sensors. Lambda sensors are continuously experiencing high temperatures – courtesy of the exhaust gases. The vibration and heat can damage the connections and wirings inside. Furthermore, the oxygen sensor can become faulty due to random decay.

Q. What Happens With A Damaged O2 Sensor?

Fortunately, a damaged oxygen sensor does not cause serious problems. You can even drive with one. Whether we recommend it, it is a whole other story. After all, this condition can hamper the condition of your engine’s parts, in the long run, not to mention overall vehicle performance.

Q. Although My Ford Can Be Driven Fine, It Keeps Displaying The P1131 Code. What Should I Do?

This can result from a malfunction of any parts involved in diagnostics. Your Ford will drive fine initially because the problem is still in its beginner phase and can exacerbate over time.

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