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P1131 Ford: Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 1

The P1131 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 F150, Ford Escort, and even Ford Mustang. In this article, we will dive into its meaning, diagnosis, and fixes.

P1131 Ford Code Definition – What Does It Mean?

General Code Definition

P1131 trouble code on Ford vehicles is defined as “Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 1”.

The code P1131 indicates “Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 1”

However, the p1131 is a manufacturer-specific trouble code, which means other brands will have different definitions.

Meaning Of P1131

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 0.5 as the crossover point. The engine is said to be running rich if the reading is above 0.5, while a reading below 0.5 indicates a lean condition. For Ford vehicles, the P1131 code will be displayed if the system isn’t switching between the 0-1volt ranges as much as it should.

Symptoms Of P1131 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 P1131 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 p1131 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 P1131
OBD2 scanning tool

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 p1131 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 p1131 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 P1131 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 P1131 Ford Code

  1. P1151 – Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 2
  2. P1137 – Lack of Heated Oxygen sensor switches Bank 1 sensor 2 indicates lean
  3. P1157 – 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 p1131 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 p1131 related issues can cause serious damage to other parts of the vehicle.

Simply put, it is not recommended to drive a vehicle with the p1131 code.

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

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