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P1131 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


P1131 is an OBD-II Code that refers to a Lack of Upstream to the Heated Oxygen Sensor Switch Sensor which Indicates Lean Bank 1.


A P1131 code is quite common in Ford vehicles and the result of the ECM (Engine Control Module) receiving a signal from the Lambda (or the oxygen) sensor. This sensor detects the levels of oxygen in the exhaust gases. The PCM (Powertrain Control Module) controls whether the engine is running on a lean or rich fuel mixture based on these results. When the engine runs on less fuel or too much air, a code is generated because of a lean running condition.


Common causes for this code include:

  • Vacuum leakage in engine
  • Leakage of the fuel injector or pressure regulator
  • Bank 1 of the In-Line Oxygen Sensors is malfunctioning
  • The MAF sensor may be dirty or non-functional.
  • Fuel pressure is not within set parameters


The symptoms of a P1131 code are: 

  • The Check Engine light turns on
  • Slower acceleration than usual
  • The engine frequently misfires
  • Poor gasoline efficiency
  • The vehicle is unable to start


To diagnose a P1131 DTC code, a technician would:

  1. Start the engine and allow it to idle
  2. Monitor all of the oxygen sensors readings on a diagnostic scanner while observing the oxygen sensor voltages. 
  3. If any voltage is out of range or if there are no changes in that voltage, then the problem is with the sensor itself not providing a signal from that particular cylinder. 
  4. Then a replacement O2 sensor must be installed before attempting a retest.
  5. If none of the sensors are showing a voltage that is out of range, then the problem must be elsewhere in the exhaust system or with malfunctions in sensors upstream from any current oxygen sensor.

Common mistakes

The following are common mistakes when diagnosing the trouble code P1131:

Failing to check for freeze frame data. 

Performing a retest of the system after replacing an oxygen sensor without performing any further diagnosis. 

Replacing parts unnecessarily without properly testing and diagnosing.

How serious is this?

A P1131 code is not a serious problem and does not cause any major issues. However, if neglected for too long, these can eventually result in significant damage to other important components.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Replace any leaking or faulty engine air-intake hoses.
  • Check and possibly repair any problems with the distributor assembly, such as a rotor or cap.
  • Check the fuel supply for any faults.
  • Tighten the Spark Plugs Connecting Wires
  • Check the Oxygen Sensor’s wiring and connections for any problems.

Related codes

A P1131 is related to and may be accompanied by the following codes: 

P1137 – Downstream Heated Oxygen Sensor 1 Malfunction

P1151 – System Too Lean (Bank 1)

P1157 – Air/Fuel Ratio (A/F) sensor 1

P0171 – System Too Lean (Bank 1)

P0174 – System Too Rich (Bank 2)

P0300 – Random Misfire

How much does it cost to fix the P1131 code?

The repair cost of a P1131 code depends on the severity of the code. If it’s a minor or major repair, the latter will be more expensive because you have to pay for labor costs and possibly even part replacements.


In conclusion, the P1131 code is relatively minor and is not a significant problem in the long-term. However, it’s best to take care of this issue as soon as possible because ignoring it can result in further damage to components within the vehicle.

P1131 Code – What Does It Mean & How To Fix It