Fault in the upstream (pre-catalyst) bank 1 oxygen-sensor signal circuit. The flat-six runs wideband/heated O2 sensors that suffer heater and signal ageing; a bad upstream sensor skews fuel trims. Note a lean-adaptation code (P1128/P0171) is usually an intake leak, not the sensor - confirm the sensor is truly at fault.
SYMPTOMS
- Check-engine light
- Rough running
- Reduced fuel economy
- Failed emissions
LIKELY CAUSES
- Aged or contaminated upstream O2 sensor
- Wiring/connector fault
- Exhaust leak upstream of the sensor
- Actual lean/rich condition skewing the signal
DIAGNOSIS STEPS
- Read the upstream O2 signal live for switching activity
- Inspect the sensor connector and wiring
- Check for exhaust leaks near the sensor
- Rule out a real mixture fault (trims) before replacing
- Replace the sensor if lazy/dead
RELATED PARTS
- Upstream O2 sensor (Bank 1 Sensor 1)