Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Circuit Description
This test determines if the heating element in the rear oxygen sensor is working properly. Oxygen sensor voltage is produced when the sensor reaches operating temperature and when the oxygen content of the exhaust stream is different than that in the atmosphere. The heating element is energized through the ignition signal circuit, when ignition is turned on, allowing sensor to reach operating temperature quickly. This in turn allows the PCM to enter into "closed loop" quicker. The voltage that the PCM measures on the oxygen sensor high and low inputs is called bias voltage. When bias voltage takes too long to vary (greater or less than bias voltage) after ignition is turned on, DTC will set.
Conditions for setting DTC:
- DTCs P0101, P0102, P0103, P0117, P0118 and P0134 not set.
- ECT dropped at least 50°F (10°C) from end of the last ignition cycle to the beginning of this ignition cycle.
- Ignition voltage 11 volts or greater.
- Average rear HO2S bias voltage 352-546 mV.
- Engine did not stall during test.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.