What does it mean?
Here the fourth oxygen sensor on bank 1 is still working electrically, but its output is offset toward the lean side of the range and will not come back. The module compares the sensor's average voltage and its switching pattern against what the commanded mixture should be producing at that point in the exhaust; when the signal stays biased low while everything else says the mixture is correct, the sensor itself is judged to be reporting falsely lean. This is different from a low-voltage circuit fault because the signal is still inside the electrical window and may still move a little. The distinction matters because a biased sensor can push the module into adding fuel it does not need, so the fault is treated seriously even though the light may be the only outward sign.
Symptoms
- MIL on, sometimes together with lean bank codes if the module has been chasing the false reading
- Positive fuel trims and increased fuel consumption on applications that trim from this sensor
- Hesitation or a slight surge under light load
- Catalyst monitor never completes and an emissions test cannot be passed
- A faint exhaust leak sound under the floor when a leaking joint is the real cause
Likely causes
- Aged sensing element that has drifted lean, typical after long service life
- Contamination of the element with silicone, oil ash or fuel additive residue
- Small exhaust leak upstream of the sensor letting fresh air into the sample, which is very common at flex joints and old clamps
- High resistance in the signal-return path, which pulls the apparent voltage down
- Weak sensor heater keeping the element just below the temperature it needs for a correct output
- A genuinely lean exhaust at that point caused by low fuel delivery or an unmetered air leak
- Wrong or low-grade replacement sensor with an offset output characteristic
How to fix it
- Graph sensor 4 against the upstream sensor and against fuel trims; a genuine lean mixture moves everything together, a biased sensor stands alone
- Do a propane or smoke check around the exhaust joints ahead of the sensor and inspect the flex pipe and the bung threads
- Measure the voltage drop on the signal-return circuit while the engine runs; more than a few tens of millivolts means a bad return
- Check heater operation and confirm how quickly the sensor becomes active after a cold start
- Force a short rich command with the scan tool if the platform supports it and see whether the sensor can still reach its rich threshold
- Rule out a real fuel delivery shortfall by checking fuel pressure and injector performance
- Replace the sensor only after the exhaust and return circuit are proven, then confirm the bias is gone in live data