What does it mean?
Sensor 3 in bank 1 sits furthest downstream in that bank's exhaust, after the additional catalyst section on vehicles that carry three sensors on a bank. A healthy oxygen sensor changes state quickly when the exhaust gas swings from rich to lean, and the module measures that transition time during a controlled fuel test. This code is stored when the sensor needs longer than the allowed period to move from a rich indication to a lean one. It is a judgement about response speed rather than about voltage limits, so a slow sensor can still show perfectly plausible values on a static data display.
Symptoms
- MIL on while the vehicle otherwise drives normally
- Slightly increased fuel consumption
- Emissions readiness monitors that will not complete
- Occasional light hesitation on throttle transitions
- Other oxygen sensor or catalyst efficiency codes appearing over time
- Failed emissions test despite no obvious driveability complaint
Likely causes
- Aged sensor element with a slow switching characteristic after high mileage
- Contamination of the sensing tip by oil, coolant or the wrong sealant used on a previous repair
- Exhaust leak ahead of the sensor letting outside air blur the transition
- Weak sensor heater circuit, so the element runs below its ideal temperature
- Soot and deposits on the element after a long period of rich running
- High resistance in the signal or ground circuit slowing the reading the module sees
- Sensor loose in its bung so it is not fully exposed to the gas stream
How to fix it
- Graph the sensor voltage through a decelerating throttle and then a reopening event, and compare its transition speed with the other sensors on the vehicle
- Check the freeze frame for the load and temperature at which the test ran, and repeat those conditions on a road test
- Inspect the exhaust between the engine and the sensor for leaks at flanges, clamps and welds
- Verify the heater circuit for supply voltage, ground and correct resistance
- Look for the reason the element became contaminated, such as oil consumption, a coolant leak into the exhaust or an unsuitable sealant
- Measure the signal and ground circuits for voltage drop and inspect the connector for corrosion
- Replace the sensor with a correct specification unit once the wiring and exhaust integrity are proven, then verify the transition speed after the repair