P0166 Code: O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3)

What the P0166 code means

The bank 2 sensor 3 oxygen sensor is the lambda sensor on bank 2 (the side opposite cylinder 1, found only on V and boxer engines), located after the second catalytic converter (sensor 3). The computer uses it because it watches the second converter on vehicles that have two in series.

The P0166 code is stored when the signal sits fixed near 0.45 V, the bias voltage the computer applies to the sensor when the sensor generates nothing. It means the sensor is not working: an open circuit, a cold sensor because its heater does not work, or a sensor that is completely worn out.

Because this sensor sits after the catalytic converter, a failure usually does not affect how the vehicle drives. It does, however, keep the system from watching the converter, and you will not pass an emissions inspection until it is fixed. Look up how O2 sensors are tested if you need the step-by-step procedure.

P0166 code symptoms

• Check engine light on.
• Usually no driving symptoms.
• Sensor or catalytic converter monitors incomplete: you will not pass the emissions inspection.
• Other codes for the same sensor or for catalytic converter efficiency may show up.

Likely causes of the P0166 code

1. Failed sensor heater. The sensor never reaches its operating temperature.
2. Open signal circuit: a cut wire or a loose connector.
3. Sensor completely worn out.
4. Sensor ground interrupted.
5. Large exhaust leak that cools the sensor.

How to diagnose the P0166 code step by step

1. Read the sensor live data with the engine warm: a healthy upstream sensor swings between 0.1 and 0.9 V several times a second, while a healthy downstream sensor stays almost steady between 0.5 and 0.8 V.
2. Look for exhaust leaks between the engine and the sensor: with the engine cold, listen for puffing or hold your hand near the joints (without touching them).
3. Inspect the wiring near the exhaust and the connector for oil, melted insulation or corroded pins.
4. Run a response test: snap the throttle open (the signal should go rich, near 0.9 V) and then close it (it should go lean, near 0.1 V). A slow response points to an aged sensor.
5. Check the fuel trims. If they are beyond plus or minus 10 percent, the sensor may be warning you about a real mixture problem.
6. Check the heater. A cold sensor responds poorly even when the element is healthy.
7. Check continuity of the signal and ground wires all the way to the computer.

How to fix the P0166 code

• Repair exhaust leaks before replacing the sensor.
• Repair burned wiring and secure it away from the exhaust.
• If the sensor is contaminated, find and fix the source (oil, coolant, rich mixture) before you install a new one.
• Replace the sensor with one of the correct specification (type, number of wires and wire length).
• Use anti-seize on the threads only, never on the tip of the sensor.
• Clear the codes, drive a full cycle and confirm the sensor monitor reaches ready status.

Technical reference data

• Conventional zirconia sensor: 0.1 V (lean) to 0.9 V (rich).
• Bias voltage with no activity: about 0.45 V.
• Approximate heater resistance: 2 to 20 ohms.
• Operating temperature: above 300 °C (570 °F).
• Usual thread: M18 x 1.5; approximate torque 40 to 50 N·m (about 30 to 37 lb-ft).
• Approximate values only: check the data for your own engine.

Brand-specific notes

On inline four-cylinder engines only bank 1 exists, so bank 2 codes should not show up.

P0166 on Toyota and Lexus

Sensor 1 is usually a wide-band (air-fuel ratio) sensor, and the equivalent codes are P2195 to P2199 and P2238 to P2243.

P0166 on GM, Toyota and Volkswagen

Sensor 3 locations are common on some models with two catalytic converters in series.

Frequently asked questions about P0166