1, 2 and 3 Digit Codes: 155

Scan Tool 155 (MIL 21):

Trouble Code Number
- Scan tool 155
- General scan tool P131
- MIL 21

Fault Message
- Upstream HO2S Shorted to Ground

Background
- When the heated oxygen sensor temperature is low, the sensor has the same electrical characteristics as an insulator.
- The heated oxygen sensor output signal line is restricted to 5V. That is why the line voltage is approx. 5V when the heated oxygen sensor temperature is low.
- If the heated oxygen sensor output signal line is grounded, its voltage will become low.

Range of Check
- Engine coolant temperature when the engine starts: 49°C (12O°F) or less
- Within three seconds after the engine starts

Set Condition
- The heated oxygen sensor output signal line is 0.16V or less.

Probable Cause
- Upstream heated oxygen sensor failed
- Wiring harness and connectors tailed
- PCM tailed