Meaning of a P0031 OBD-II Code
Skip to the good stuff:
The technical definition of the P0031 code is: Oxygen Sensor Heater Control Circuit Low Bank 1 Sensor 1
Based on the code definition, it has something to do with O2 sensor (oxygen sensor) in bank 1, which is usually located in front of the catalytic converter.
This should not be confused with oxygen sensor #2, which is located behind the catalytic converter. Also, the P0031 OBD2 trouble code is similar to P0052, P0051, and P0032.
But this problem doesn’t indicate a faulty O2 sensor. The generic fault code specifically relates to heater circuit in the sensor. The ECM utilizes the readings from the O2 sensor to adjust the air/fuel ratio based on the amount of oxygen in the exhaust and outside air. And since the O2 sensor is less effective if the exhaust gas temperature is low, it relies on a heater to get better readings.
George Melnik did a great job troubleshooting this DTC on a Subaru.
What are the causes of the P0031 fault?
The causes will vary from a:
- Poor Ground
- Faulty connector
- Broken heater circuit
- Bad O2 sensor
- Failing ECM
If there is a short or open circuit in the heater, the problem can be attributed to faulty or broken wiring. The worst case scenario can be traced to a broken ECU, but this is highly unlikely even when working with an older vehicle.
What are the symptoms of a P0031 DTC?
The first symptom is a check engine light in the console. Once the ECM detects a low heater circuit, the ECM will enter into failsafe mode. It will only revert to normal mode once the ignition is turned off. But the system will again enter into failsafe mode until the problem is fixed.
If the ECM is in failsafe mode and the engine is running, you will notice:
- drivability issues
- poor acceleration
- bad idling
How much does it cost to fix the P0031 OBD2 Trouble Code?
The answer depends on the severity of the problem.
The biggest mistake you can make is to replace the O2 sensor entirely without inspecting the related wiring and connectors. If the problem is only caused by faulty wiring, the problem can be fixed without spending more than $20 to $30.
But if the O2 sensor needs to be replaced, you can expect to spend $250 to $400 in parts and labor depending on the make and model of the vehicle. In most cases, both O2 sensors will need to be replaced to maintain consistent readings.
A good mechanic should check for 12 volts at the heater circuit by unplugging the sensor and checking the wiring connector. Don’t spend too much on this tool, just grab a $10 tester at Harbor Freight!
And for good measure, the resistance of the heater circuit should be checked before replacing the sensor altogether. Whenever replacing critical engine sensors, always insist on using the OEM variety to avoid problems later on.