Advanced Diagnostics

Advanced Diagnostics - DTC P0155: Front Air/Fuel Ratio (A/F) Sensor (Bank 2, Sensor 1) Heater Circuit Malfunction






General Description
A heater for the sensor is embedded in the front air/fuel ratio (A/F) sensor (bank 2, sensor 1), and it is controlled by the powertrain control module (PCM). It is activated and heats the sensor to stabilize and speed up the detection of oxygen content when the exhaust gas temperature is cold. The PCM monitors the front A/F sensor heater output (return check). If the return signals do not meet the command value (for heater activation) for a set time, the PCM detects a malfunction and stores a DTC.

Monitor Execution, Sequence, Duration, DTC Type, OBD Status






Enable Conditions






Malfunction Threshold
One of these conditions must be met for at least 5 seconds:
- No return signal HIGH is detected when the PCM output duty is less than 5 %.
- Return signal does not change when the PCM output duty is 5 %* or more.

*: PCM output duty is limited to 95 % when it exceeds 95%.

Driving Pattern
Start the engine. Hold the engine speed at 3,000 rpm without load (in P or N) until the radiator fan comes on, then let it idle for at least 5 seconds.

Diagnosis Details

Conditions for setting the DTC
When a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC
The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command or by disconnecting the battery.