Emissions

P0166 — O2 Sensor Circuit No Activity (Bank 2, Sensor 3)

Severity: low Safe to drive

What P0166 means

The engine control module monitors the oxygen sensor mounted downstream of the catalytic converter on the Bank 2 side (Sensor 3). Its signal normally changes with the oxygen content of the exhaust. With P0166, the signal stays flat or inactive for too long, so the module logs a no-activity fault in the circuit.

Symptoms

  • Check Engine light on
  • Slightly higher fuel consumption
  • Occasional unstable idle
  • Possible emissions test failure
  • Often no other noticeable symptoms

Possible causes

  1. Failed Bank 2 Sensor 3 oxygen sensor
  2. Damaged wiring or corroded connector terminals
  3. Exhaust leak near the sensor
  4. Blown fuse for the sensor heater or a poor ground
  5. PCM fault (rare)

How to diagnose

  1. Scan for codes and freeze frame data, clear the code and see if it returns
  2. Inspect the sensor, harness and connector for breaks, chafing, heat damage and corrosion
  3. Check the exhaust for leaks near the sensor
  4. Check the fuse, heater power supply and ground
  5. View live sensor voltage and heater status on a scan tool
  6. Review fuel trims to rule out mixture problems
  7. Test the wiring between the sensor connector and PCM for opens and shorts
  8. Replace the sensor if the wiring checks out

Common fixes

  • Replace the Bank 2 Sensor 3 oxygen sensor
  • Repair the wiring or replace the connector
  • Fix the exhaust leak
  • Replace the fuse or repair the ground
  • Reflash or replace the PCM (rare)

Questions and answers

What does P0166 mean?

The PCM sees no change in the signal from the Bank 2 Sensor 3 oxygen sensor, which sits downstream of the catalytic converter. The most common causes are the sensor itself or its wiring.

Can I drive with P0166?

Usually yes for a short time. Don't put off the repair, though, as the light will stay on and you may fail an emissions test.

Should I replace the sensor right away?

No. Check the wiring, connector, fuse and exhaust for leaks first. Replace the sensor if those are fine.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.