P1166
Air/Fuel Ratio Sensor 1 Heater Circuit Malfunction
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The P1166 code on Subaru vehicles indicates that the ECM has detected a malfunction in the heater circuit of the upstream Air/Fuel Ratio sensor (Bank 1, Sensor 1). This sensor contains a built-in electric heater that quickly brings it to its optimal operating temperature of around 1,200°F. Without a functioning heater, the sensor relies on exhaust heat alone and takes significantly longer to become active, especially during cold starts. During this extended warm-up period, the engine runs in open-loop mode with a richer fuel mixture, resulting in reduced efficiency and higher emissions.
The most common cause is simply a worn-out heater element within the sensor itself. A/F sensors are wear items and the internal heater typically lasts 80,000 to 120,000 miles before failing. Less commonly, the issue may be a blown fuse, damaged wiring, or a corroded connector — all of which are worth checking before replacing the sensor.
This code is not urgent and the vehicle can be driven safely, but you should address it within the next month or so. Prolonged driving with a cold-start rich condition wastes fuel and can accelerate catalytic converter wear. Start by checking the heater circuit fuse, which is the cheapest possible fix. If the fuse is fine, replacing the A/F sensor is a moderate DIY job that requires an oxygen sensor socket.
Severity
Symptoms
- •Check engine light illuminated
- •Slightly decreased fuel economy, especially during cold starts
- •Engine may run rough for longer during warm-up
- •Possible rough idle when engine is cold
- •Increased exhaust emissions or smell
Likely Causes
The internal heater coil in the air/fuel ratio sensor can burn out over time. The heater brings the sensor to operating temperature quickly, and when it fails, the sensor takes much longer to produce accurate readings.
The heater circuit wires are exposed to extreme exhaust heat. Insulation breakdown or wire breakage causes an open circuit that prevents power from reaching the heater element.
A blown fuse or faulty relay in the A/F sensor heater circuit cuts power to the heater. This is an easy check and an inexpensive fix if it is the root cause.
Corroded or loose pins at the sensor plug introduce resistance into the heater circuit, preventing sufficient current flow to properly heat the sensor element.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the upstream air/fuel ratio sensor (Bank 1 Sensor 1)
- Check and replace the A/F sensor heater circuit fuse
- Repair or replace damaged wiring in the heater circuit
- Clean corroded electrical connections at the sensor plug