DTC P0156: Ho2S Circuit - Bank 2, Sensor 2: Description

WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The Heated Oxygen (HO2S) bank 2 sensor 2 is threaded into the front exhaust pipe directly behind the cylinder 2, 4, 6 pre-converter (cylinders 2, 4, 6 are closest to radiator). The sensor is used by the ECM to determine the oxygen storage capability of the pre-converter, fuel trim biasing (time held lean or rich) and certain techniques used to diagnose DTCs associated with the pre-catalyst oxygen sensor. The sensor is an electrical source that responds to oxygen content out of the pre-converter. When the sensor reaches approximately 600°F (316°C), it is fully functional and produces a voltage based on the difference in oxygen between the atmosphere and exhaust gas. The ECM sends a 450 mV bias voltage on the signal line, which is pulled up through high resistance. When the HO2S bank 2 sensor 2 is cold, it produces no voltage and has extremely high internal resistance. The internal resistance of the sensor is much greater than the resistance of the bias pull-up resistor. However, when the sensor heats up, it produces voltage that overrides the bias voltage. Under normal conditions, low sensor voltage means high oxygen content (lean air/fuel mixture) and vice versa. Normal sensor readings will be around 500-800 mV under steady cruise. DTC P0156 will set if the HO2S bank 2 sensor 2 signal voltage rises whenever its heater is being commanded off for a certain number of times.