P0166
O2 Sensor Circuit No Activity Detected Bank 2 Sensor 3.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- 66
- FaultThe specific fault in that area
What it means
P0166 means the engine control unit has detected no activity from the oxygen sensor at Bank 2, Sensor 3, which is the third sensor on the Bank 2 side, normally fitted downstream of the catalytic converter. The control unit saw a signal that stayed flat, absent or outside the expected range when it should have been switching or responding. Sensor numbering and layout vary by model and engine, so confirm which sensor is Bank 2 Sensor 3 on the vehicle in question.
Symptoms
- Engine warning light on
- Usually no noticeable change in driveability
- Possible slight rise in fuel consumption on some models
- Other oxygen sensor or catalyst monitor codes stored alongside
- Failed emissions test or readiness monitors not completing
Possible causes
- Failed or aged oxygen sensor (Bank 2 Sensor 3) giving no signallikely
- Damaged, chafed or melted sensor wiring, often near the exhaustlikely
- Corroded, loose or contaminated connector at the sensor or in the harnesslikely
- Sensor heater circuit fault, blown fuse or relay problem (depends on model)possible
- Exhaust leak near the sensor affecting readingspossible
- Sensor contaminated by coolant, oil or fuel additivespossible
- Engine control unit fault or software issuerare
How to diagnose
- Read all stored and pending codes and note freeze frame data, including engine temperature and load when the fault set.
- Confirm which physical sensor is Bank 2 Sensor 3 using the manufacturer's layout for that engine.
- Visually inspect the sensor, wiring and connector for heat damage, chafing, corrosion, water ingress or poor locking.
- Check for exhaust leaks around the sensor and the pipework upstream of it.
- Check the relevant fuses and relays for the sensor heater circuit where fitted.
- With the engine warm, view live sensor data and check whether the signal is flat, stuck or responding to changes in throttle.
- Test the sensor heater resistance, supply, earth and signal wiring continuity against the manufacturer's specification.
- Check for other codes that point to a wider fuel, misfire or catalyst issue before replacing parts.
How to fix
- Replace the Bank 2 Sensor 3 oxygen sensor if it is confirmed faulty.
- Repair or replace damaged wiring and terminals in the sensor circuit.
- Clean, dry and refit or renew a corroded or loose connector.
- Replace a blown fuse or faulty relay after finding the cause of the failure.
- Repair any exhaust leak affecting the sensor reading.
- Resolve any underlying oil, coolant or fuel contamination issue before fitting a new sensor.
Can I keep driving?
The car can normally be driven while you book it in, as this sensor usually has little effect on running. Have it checked reasonably soon to avoid a failed emissions test and to rule out other faults.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.