P0166
O2 Sensor Circuit No Activity Detected Bank 2 Sensor 3
Oxygen sensor / emissions
Generic OBD-II
Medium
What it means
P0166 means the ECM sees no switching activity from the third oxygen sensor on bank 2 — a far-downstream sensor used on some vehicles to monitor emissions. A flat, inactive signal usually points to a dead sensor, an open in its wiring or heater circuit, or an exhaust leak. The car typically drives normally, but the fault can cause an emissions-test failure.
Medium
Can I keep driving? Safe to drive — a post-catalyst oxygen-sensor fault won't strand you; the car runs normally but may fail an emissions test until it's fixed.
Common symptoms
What you'll notice
- Check-engine light on
- Often no noticeable change in how the car drives
- Slightly reduced fuel economy
- May fail an emissions/readiness test
Common causes
Most likely first
- Failed or dead O2 sensor (bank 2, sensor 3)Likely
- Open wiring or loose connector in the sensor circuitCommon
- Failed sensor heater or blown heater fuseCommon
- Exhaust leak near the sensor; rarely, an ECM faultLess often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and check the bank 2 sensor 3 signal in live data — it should switch once warm.
- Inspect the connector and wiring for opens, corrosion, or a loose fit; check the heater fuse.
- Look for exhaust leaks near the sensor that could stop it from reading properly.
- Replace the sensor if it stays inactive with good wiring and power. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0166?+
Yes. This far-downstream sensor monitors emissions rather than controlling fueling, so the car drives normally — but it may fail an emissions test until repaired.
Can I just clear P0166?+
You can, but if the sensor is dead or its circuit is open, the code returns after the sensor should have warmed up and switched.
Related codes
Read & clear P0166 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX