What does it mean?
This code is stored when the bank 1 sensor 4 output stays offset toward the rich side of its range while the rest of the fuel control data says the mixture should be near stoichiometric. The module watches the sensor's average level and how it responds to commanded mixture changes; a signal that holds high and refuses to fall through its lean threshold is classified as biased or stuck rich. Because the reading is still electrically plausible, this is a performance fault rather than a high-voltage circuit fault. A sensor that reports falsely rich can make the module subtract fuel it should not, so both the sensor and the possibility of a genuinely rich exhaust have to be examined. The catalyst monitor is suspended until the signal behaves normally again.
Symptoms
- MIL on, frequently with rich mixture or negative fuel trim codes
- Black or sooty exhaust deposits and a strong fuel smell when the mixture really is rich
- Higher fuel consumption and reduced power
- Rough idle or occasional misfire if the mixture is genuinely over-rich
- Catalyst monitor incomplete and a failed emissions inspection
Likely causes
- Element contaminated by burnt oil, coolant or fuel additives, which typically biases the output rich
- Aged sensor whose reference air path has become blocked by dirt or by a sealed connector, so it reads high
- Genuinely rich exhaust reaching the rear sensor from a leaking injector, high fuel pressure or a failed upstream sensor
- Leakage current from the heater circuit into the signal wire through a damp or corroded connector
- Poor signal return that lifts the apparent voltage
- Excessive oil consumption past valve seals or rings feeding oil into the exhaust
- Substandard replacement sensor with a rich offset
How to fix it
- Compare sensor 4 with the upstream sensor and with fuel trims to decide whether the exhaust is truly rich or only this sensor says so
- If the whole system reads rich, test injector leakage, fuel pressure and the upstream sensor before touching sensor 4
- Inspect the sensor connector for moisture and corrosion that could feed heater voltage into the signal pin
- Check the signal return for continuity and offset against module ground
- Look at the reference air path of the sensor; a connector packed with grease or dirt can block it and bias the reading
- Check for oil or coolant consumption that would poison the element
- Replace the sensor last, then verify that it can sweep fully lean and rich during a road test