P0166 – O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3).

What the code means

P0166 sets when the ECM sees no voltage switching from Bank 2, Sensor 3 for a certain time.

Bank 2 = side of engine without cylinder #1

Sensor 3 = rear O2 sensor, after the catalytic converter (on vehicles that have 3 sensors per bank)

This sensor mainly monitors catalytic converter efficiency, not fuel trim.

Common causes (most to least common)

1. Failed O2 sensor (most common)

2. Open or shorted wiring near exhaust (heat damage)

3. Sensor heater circuit failure

4. Exhaust leak ahead of sensor

5. Connector corrosion or loose pins

6. ECM issue (rare)

Quick checks (10–15 minutes)

1. Visual inspection

Melted wires?

Sensor harness touching exhaust?

Connector loose or corroded?

2. Check exhaust

Leak before or at the sensor = false no-activity reading

Electrical diagnostics (recommended)

1. Live data test (scan tool)

Monitor B2S3 O2 voltage

Normal: slow movement around 0.6–0.8V

Faulty: stuck at 0.0V, 0.45V, or flatline

➡ If flatlined → sensor or wiring issue

2. Heater circuit test (key ON)

Unplug sensor

Measure resistance across heater pins

Typical: 5–14 ohms (check service data)

Infinite or 0 ohms = bad sensor

3. Signal wire test

Back-probe signal wire

Look for 0.1–0.9V fluctuation

No voltage change → sensor or signal wire open

Common repair

✅ Replace Bank 2 Sensor 3 O2 sensor

Labor: 0.5–1.0 hr

Part cost:

Aftermarket: $60–$120

OEM: $120–$250

Important notes for customers

This usually won’t affect drivability

Can cause failed emissions test

Leaving it unfixed can mask catalytic converter issues.

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