P0166 Oxygen sensor: no signal (bank 2, sensor 3)

By: Diagno editorial team · Updated: 2026-10-15

P0166 means: Oxygen sensor: no signal (bank 2, sensor 3). The oxygen sensor measures oxygen in the exhaust and the computer sets the fuel mixture by it. OK to drive, but have it checked soon. The repair usually costs about €60–250 in Estonia. The most common cause: connector loose, damp or corroded.

What it means

The oxygen sensor measures oxygen in the exhaust and the computer sets the fuel mixture by it. Sensor 1 sits before the catalytic converter, sensor 2 after it. The computer found the signal wrong, slow or missing. The computer gets no signal at all from the part. Bank 2 is the side of a V engine without cylinder 1.

What you may notice

• Engine light on
• Higher fuel consumption
• Idle may be uneven
• Emissions may fail the inspection

What could be wrong

1. 1Connector loose, damp or corroded
2. 2Wire broken or chafed
3. 3Faulty oxygen sensor
4. 4Engine computer fault (rare)

Repair difficulty: Moderate

What you can check yourself

First look at the connector and wiring: anything loose, melted or green with corrosion? Listen for an exhaust leak ahead of the sensor. Check that the sensor wire is intact and the connector is clipped in. You can do the simpler checks yourself.

How we fix it

1. We read the fault data on the diagnostic tool and check whether the fault is constant
2. We measure the circuit's supply, ground and signal
3. We repair the wire or replace the connector
4. If the wiring is fine, we replace the oxygen sensor

Standard SAE description: O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 3)