P0166 Code: O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3)
🟠 Moderate severity · drivable
Drivable, but the sensor is not reporting, so fuel economy and emissions may suffer until it's fixed.
What P0166 means
The oxygen sensor at bank 2 sensor 3 (a downstream sensor after the catalytic converter) is not sending a changing signal. The computer expects the sensor voltage to swing as exhaust oxygen changes, but it stays flat. This code usually means the sensor or its wiring has failed.
Symptoms
- Check engine light is on
- Failed emissions test
- Slightly worse fuel economy
- Usually no noticeable driving symptoms
Common causes
- most common: Faulty downstream oxygen sensor
- common: Wiring or connector damage at the sensor
- common: Exhaust leak near the sensor
- less common: Contaminated sensor from oil or coolant
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Replace the bank 2 sensor 3 oxygen sensor | Yes | $60–200 |
| Repair or replace damaged wiring and connectors | Yes | $10–60 |
| Fix an exhaust leak near the sensor | Mechanic | $50–200 |
Frequently asked questions
Can I drive with a P0166 code?
Yes, the car is usually safe to drive. It mainly affects emissions monitoring and fuel economy, not safety.
What does "no activity detected" mean?
The oxygen sensor's voltage is stuck at one level instead of switching, so the computer sees no change in the exhaust reading.
What is the most common cause of P0166?
A failed downstream oxygen sensor, often followed by wiring problems or an exhaust leak near the sensor.
Related codes
P0162
P0162 Code: O2 Sensor Circuit Malfunction (Bank 2 Sensor 3)
P0165P0165 Code: O2 Sensor Circuit Slow Response (Bank 2 Sensor 3)
P0146P0146 Code: O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 3)
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.