What does it mean?
P1166 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Air Fuel Sensor (Bank 1 Sensor 1) Heater Circuit Malfunction .
Symptoms
- Check engine light on, often within a minute or two of a cold start
- Long delay before the engine enters closed loop, with rich running while cold
- Slightly rough idle and extra fuel consumption during warm-up
- Emissions readiness monitors will not complete
- No obvious symptoms once the engine is fully warm, since exhaust heat keeps the sensor hot
Likely causes
- Open or shorted heater element inside the sensor, the most common cause
- Blown fuse or faulty relay feeding the sensor heater circuit
- Chafed, burnt or broken heater wiring where it runs near the exhaust
- Water, corrosion or oil in the sensor connector raising resistance
- Poor heater ground connection or a corroded ground eyelet
- Failed heater driver inside the PCM (powertrain control module)
How to fix it
- Check the fuse and relay that supply the sensor heater
- Unplug the sensor and measure heater element resistance across its two heater pins; an open or near-zero reading condemns the sensor
- With the ignition on, verify battery voltage at the heater supply pin of the connector
- Back-probe the heater control wire and confirm the module is pulling it to ground
- Inspect the harness where it passes near the exhaust for melted insulation and check the connector for corrosion
- Repair wiring faults first, then replace the sensor if the heater element itself is open
- Clear the code and confirm the heater monitor completes on a cold start