Upstream HO2S Shorted to Ground

Code Number
Scan tool 155
General scan tool P0131
MIL 21

Description
Upstream HO2S Shorted to Ground

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

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 (120°F) or less
- Within three seconds after the engine starts

Set Conditions
The heated oxygen sensor output signal line is 0.1 6V or less.





Diagnosis