1, 2 and 3 Digit Codes: 124

Scan Tool 124 (MIL 21):

Trouble Code Number
- Scan tool 124
- General scan tool P0155
- MIL 21

Fault Message
- Front Bank Upstream HO2S Heater Failure

Background
- The PCM continues to monitor this system after the key is OFF.
- The PCM waits some periods to allow the heated oxygen sensors to cool.
- Once the PCM calculates that the heated oxygen sensor is cooled, it energizes the MFI relay (ASD relay) and tests for the heated oxygen sensor operation.

Range of Check
- 5 seconds or more after turning the ignition switch to OFF.
- Battery positive voltage: 10 V or more
- The heated oxygen sensor output line voltage rises by 0.49 V or more within 144 seconds after turning the ignition switch to OFF.
- The initial rise of the output line voltage is less than 1.57 V.

Set Condition
- Front bank upstream heated oxygen sensor circuit does not show the appropriate decrease of voltage during the test.

Probable Cause
- Front bank upstream heated oxygen sensor heater open circuit
- Wiring harness and connectors failed