P0031: O2 Sensor → Heater Control Circuit → Low (B1S1)

P0031 is a fairly common OBD II trouble code. It’s generic, which means that it applies to any vehicle made since 1996. As far as trouble codes go, this is a relatively easy one to diagnose. It is usually caused by a bad oxygen sensor, or a wiring issue.  

 

P0031 Diagnosis

 

P0031 Definition

Bank 1 Sensor 1 Location- Bank 1 is the side of the engine with cylinder 1. If you have a four cylinder engine or inline six, it’s the only bank. Sensor 1 is in front of the catalytic converter.

The oxygen sensor’s job is to measure the level of oxygen in the exhaust so that the ECM can make changes to the air fuel ratio of the vehicle.  The oxygen sensor has a heater that helps to give it a more accurate reading.  When this heater is not operating, it can affect the way that the engine runs.  With P0031, the O2 sensor is informing you that the heater circuit resistance is lower than the normal operating range.

When the engine is cold, it has a hard time getting a good reading from the oxygen sensors.  Without a good reading, it is difficult for the ECM to determine the proper air/fuel mixture to give the engine.

 

P0031 Symptoms

P0031 is most often not accompanied by any noticeable symptoms as long as it is the only code that is currently stored in the ECM’s memory.  If there are symptoms, they’ll be:

  • Service Engine Soon Light
  • Diminished Fuel Mileage
  • Rough Running Engine
  • Raw Fuel Smell Coming From Exhaust (particularly when cold)

If the engine is running rough, it very well may be accompanied by the P0300 code.  This indicates a random misfire in the cylinders.

 

Causes

The most common cause of P0031 is a bad oxygen sensor.  It’s, not the only cause though.  A quick examination of the physical wiring harness, as well as the voltage coming from the heated O2 sensor can save you from buying a sensor that you don’t need.

 

Wiring Issue

Due to the location of the oxygen sensors, the wiring harness around them is easily susceptible to damage.  Hot exhaust can make the wiring harness brittle.  A visual inspection will often uncover exposed wiring (that’s shorted) or wiring that is broken altogether.  Pay particular attention to anywhere that the harness comes close to touching other metal, and where it is closest to the exhaust.

If the harness looks like it is intact, you’ll next want to to test and make sure that the heated oxygen sensor is getting power.  You can take a simple test light and check to see if the harness is getting voltage.  If it’s not, check the fuse.  Make sure to test the ground circuit for continuity.

If the oxygen sensor is getting power to it, you can check to see if it is getting the correct level of voltage/ohms.  Here’s a great video from Ratchets and Wrenches on how to test the wiring on a heated O2 sensor:

 

Oxygen Sensor

It is highly likely that the oxygen sensor has gone bad when you get this code.  The good news is, they are affordable and easy to replace.

 

Bad ECM

It is technically possible that the ECM has gone bad when everything seems to be working, but you still get this code anyway.  It’s not very likely though.  Here’s more on the symptoms of a bad ECM.

 

Conclusion:  P0031

This code is relatively straightforward to diagnose.  If there is anything that you would like to add, please feel free to leave a comment below.  Best of luck to you.

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