OBD-II Trouble Code Description for P0031 Subaru

HO2S Heater Control Circuit Low

What does this mean?

OBD2 Code P0031 Subaru definition:
A P0031 Subaru DTC (diagnostic trouble code) definition:the O2 sensor (oxygen sensor) located on Bank 1 in front of the catalytic converter. There is also an oxygen sensor behind the converter which is Sensor #2. This O2 sensor #1 may also be refered to as an air/fuel ratio sensor since on some vehicles it is. It detects the amount of oxygen in the exhaust gas compared to the outside air and then vehicle’s computer adjusts the air/fuel ratio going into the engine. The sensor is less effective when the exhaust gas temperature is low, so it includes a heater which is activated to help get better readings from the A/F O2 sensor. Essentially this P0031 Subaru code means that the resistance of the heater circuit is lower than normal. In most cases, that resistance level must fall below 0.8 A to trigger the DTC code. Note, this code is very similar in nature to P0032 Subaru, P0051 Subaru, and P0052 Subaru

Symptoms

Possible sumptoms of OBD code P0031 Subaru
Most likely you’ll not notice any symptoms other than the MIL (malfunction indicator lamp, a.k.a. the check engine light) will illuminate.

Causes

Possible causes of OBD code P0031 Subaru
A P0031 Subaru DTC trouble code may be caused by one or more of the following: A short or open in the heater circuit in the sensor A failed O2 sensor heater Wiring/connectors broken/frayed leading to sensor and/or relay Failed PCM/ECM

Possible Solutions

To fix a P0031 Subaru DTC code, you’ll need to do a proper diagnosis. To do that, you’ll want to inspect the wiring and connectors leading to the sensor. Also if equipped with a heater relay and fuse, you’ll want to check those as well. Use a digital volt ohm meter to: check for 12 volts at the heater circuit feed (hint: unplug the sensor and check at the wiring connector to do this measurement) check the ground circuit for continuity measure the resistance of the heater circuit (done on the sensor itself) measure the resistance and voltage of the wiring Refer to a service manual for the correct specifications (volts, ohms) for your vehicle. On some Toyota vehicles this code is triggered when the resistance of the heater circuit is below 0.8 A. With that said, a common fix for this DTC is to replace the air/fuel (O2 oxygen) sensor #1 on bank 1. Please keep in mind that OEM (original equipment) replacement sensors are recommended (from the dealer). Aftermarket sensors can be less reliable and of poorer quality (not always, but more often). There’s also a chance that replacement parts for the P0031 Subaru code may also fall under a federal emissions warranty (check with your dealer to see if it’s covered).

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