P1131 – Lack Of HO2S Switch – Sensor Indicates Lean (Ford, Lincoln, Mazda, Mercury)

By Randy Worner (Contact Me)
Last Updated 2016-11-14
ASE/ACA World Class Technician

Manufacturer Specific Definitions for P1131

Table of Contents

1. What Does Code P1131 Mean?
2. What are the common causes of code P1131?
3. What are the symptoms of code P1131?
4. How do you troubleshoot code P1131?
5. Codes Related to P1131
6. Get Help with P1131

What Does Code P1131 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 P1131 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, P1131 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 P1131?

• 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 P1131?

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

How do you troubleshoot code P1131?

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 P1131.

If the P1131 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 P1131 is about to return. If the P1131 does not return, then the connections/wiring issues were most likely your problem.

If the P1131 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 P1131 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 P1131

P1151 – Upstream Heated O2 Sensor or Fuel System Not Switching Indicating Lean Bank 2 –  Same as P1131 but for bank 2.

P0171 and P0174 – System Too Lean Bank 1 and Bank 2 –  Same diagnostic indications and steps as P1131 and P1151, but their requirements to set are different.

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