The ODB2 error code P0031 indicates that the exhaust must have an air to fuel ratio of 14.7, as instructed by the heated oxygen sensors (HO2S). The HO2S are responsible for monitoring the oxygen content of the exhaust in the vehicle, and are important in allowing the vehicle to function properly. It sends the information to the ECM to ensure the correct amount of fuel is sent to the engine.
The HO2S is kept heated to ensure that the communication between it and the ECM remains fast, which helps reduce emissions when the engine is warming up or being started.
As with most ODB2 codes, the first symptom most people will notice is that the Check Engine Light will be illuminated. The ECM will detect that the Oxygen sensor heater circuit is low, and will trigger the failsafe mode until the ignition is switched off.
Depending on the make and model of the vehicle, this failsafe mode can cause various functionality issues with the vehicle. The vehicle will continue in this failsafe mode until the issue is resolved. While the drive issues will vary, some of the most common include the engine running rough, low power to the engine and engine hesitation.
The likelihood of the error code is that the HO2S sensor for bank 1 sensor 1 is either defective or damaged. The first thing most mechanics will do is reset the code to clear it before checking if the code comes back. If it does, a faulty sensor is most likely to be the cause.
Common faults with the H2OS sensor are malfunctioning internal heater elements, or potentially a bad ground. The input or connector for the 12v battery can also be faulty, while in much rarer cases the ECM itself may be at fault.
The car will still be drivable while the error is being triggered, but you should look to have it diagnosed and repaired as soon as possible. Failure to fix the issue can cause problems with excess fuel consumption, sensor loop failure, or damage to other important engine components.
How to Check
The code is initially diagnosed through the use of the OBD2 scanner, and after detection it’s common to reset the code to see if it reappears. Should the check engine light switch back on and the error code returns, the wiring to the sensor should be inspected alongside the power and the ground. In many cases, the cause of this issue is linked to problems with the wiring, as the heat from the catalytic converter and the exhaust can cause some damage.
In high mileage vehicles, many will experience intermittent sensor problems triggered during start up or during prolonged periods of drive train stress. If the check engine light is illuminated and there’s no issues with the function of the vehicle, resetting the code can clear the error and it may not occur again.
How to Fix
As with most error codes, the first step will be to verify the code using a scanner, before resetting the code and performing a road test. If it still occurs, the steps for repair include:
A full inspection of the wiring and connectors for any signs of wear or heat damage should be carried out. If it’s in good condition and has proper voltage and ground, it may be a faulty sensor.
Placing the key in the ignition but leaving the engine off, check the battery feed to the heater element. If there’s no voltage detected, the short or open 12 volt feed circuit should be repaired by checking for any blown fuses caused by the short.
Should the feed to the battery be intact, the next step is to remove the ground control circuit from the wiring connector in the ECM and check the open circuit for resistance. If there’s infinite resistance, the open in the circuit should be repaired. If it checks out, it’s likely that the oxygen sensor is faulty and will need replaced.