P0166
O2 Sensor Circuit No Activity Detected (Bank 2, Sensor 3)
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P0166 indicates that the PCM is receiving absolutely no signal activity from the third oxygen sensor on Bank 2. Unlike a slow response code, this means the sensor is completely dead — producing no voltage fluctuations whatsoever. The PCM has lost all monitoring capability for the secondary catalytic converter on this bank.
This is rated as moderate severity because a completely non-functional sensor means the PCM has zero visibility into the secondary catalytic converter's health. On some vehicles, the PCM may adjust its operating strategy in response to the missing data, which can affect fuel economy and idle quality. The vehicle is still safe to drive, but this should be addressed within a week.
Before purchasing a replacement sensor, perform a basic visual inspection. Check that the sensor connector is plugged in securely, inspect the wiring for physical damage, and verify the sensor circuit fuse is intact. These simple checks can save you the cost of a sensor if the issue is just a loose connection or blown fuse. If the wiring and connections check out, sensor replacement is the definitive fix.
Severity
Symptoms
- •Check Engine Light is on
- •Reduced fuel economy
- •Possible rough idle
- •Elevated exhaust emissions
- •Failed emissions inspection
- •Engine may feel slightly sluggish
Likely Causes
The third oxygen sensor on Bank 2 has entirely failed and is producing no voltage output at all, leaving the PCM with a flat-line signal and no data from this sensor.
A completely severed wire or disconnected harness in the sensor circuit has cut off all communication between the sensor and the PCM.
The sensor connector has been accidentally unplugged during service work or has vibrated loose over time, completely interrupting the signal path.
A blown fuse has disabled the sensor's heater element, and without reaching operating temperature the sensor cannot produce any measurable voltage activity.
The PCM's internal circuit responsible for reading this sensor has failed, making the sensor appear completely inactive.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the Bank 2 Sensor 3 oxygen sensor
- Reconnect disconnected wiring or sensor connector
- Repair severed or broken wires in the sensor harness
- Check and replace blown fuses in the heater circuit