O2 Sensor Circuit Malfunction (Bank 1, Sensor 1)
The upstream oxygen sensor on bank 1 reports an implausible signal — typically stuck mid-range, no switching between rich and lean, or no signal at all. Closed-loop fuel control cannot run while this fault is active.
Affected components
- Oxygen sensor upstream
- Oxygen sensor wiring
Common causes
- High likelihood: Oxygen sensor aged or contaminated
- Medium likelihood: Sensor harness chafed against exhaust
- Low likelihood: Water in sensor connector
Possible symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
Related codes
Sources
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.