1, 2 and 3 Digit Codes: 126

Scan Tool 126 (MIL 21):

Trouble Code Number
- Scan tool 126
- General scan tool P0138
- MIL 21

Fault Message
- Downstream HO2S Shorted to Voltage

Background
- Downstream heated oxygen sensor requires a heater circuit for proper operation.
- The heater circuit is supplied with battery voltage through the MFI relay (ASD relay).
- A failure in the sensors heater circuit prevents it from providing accurate data to the PCM.
- If the PCM monitors more than 1.2 volts, the sensor has shorted to heater circuit voltage.

Range of Check
- 2 or more minutes after engine starts
- Engine coolant temperature: 80°C (176°F) or more

Set Condition
- Heated oxygen sensor output voltage is more than 1.2 V for 3 seconds.

Probable Cause
- Heated oxygen sensor failed
- Wiring harness and connectors failed
- PCM failed