Code Definition

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

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P0166

P0166 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0166 is stored, the module has detected a bank-specific fault, meaning it applies only to one cylinder bank of a multi-bank engine; and a sensor-related fault — the reported value from the named sensor is out of spec.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0166 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The P0166 fault code is activated when the Engine Control Module (ECM) detects no activity or insufficient activity from the oxygen sensor in Bank 2, Sensor 3. This sensor is usually located downstream in the exhaust system and may be part of vehicles with complex emission control systems or multiple catalytic converters. 'No activity' means that the sensor is not providing the expected fluctuating voltage output that reflects changes in the oxygen levels in the exhaust gas. This lack of activity can be due to a faulty sensor, issues with the wiring or connections, or an exhaust leak affecting...

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