P0156

Diagnostic Chart:

Schematic:

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 316°C (600°F), 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 the heater is being commanded Off for a certain number of times.

DTC PARAMETERS
DTC P0156 will set if HO2S bank 2 sensor 2 signal voltage rises whenever the heater circuit is being commanded Off when:
^ Condition exists for 16 out of 24 times the heater is being turned Off (heater is turned Off once every 10 seconds)
^ Exhaust pipe moisture has evaporated (calculated engine run time between 0 and 200 seconds), then engine must run for longer than 80 seconds
^ Calculated exhaust gas temperature is less than 800°C (1472°F)
^ Main relay voltage is greater than 10.5 volts

DTC P0156 diagnostic runs continuously once the above conditions have been met.

P0156 is a type B DTC.

DIAGNOSTIC AIDS
This DTC indicates that when the HO2S bank 2 sensor 2 heater is turned Off, the sensor signal voltage rises. This indicates that the heater voltage on the heater control wire is being picked up by the sensor signal wire. This coupling could be due to a short between the heater control circuit and signal, corrosion in the harness connector or current leaking internally in the sensor.