P0166 indicates the powertrain control module (PCM) has detected little or no voltage activity from the oxygen sensor located after the catalytic converter on Bank 2, position Sensor 3. The sensor signal is staying essentially flat instead of varying with exhaust oxygen content. This usually points to a problem with the sensor, its wiring, or the exhaust stream reaching the sensor, rather than the PCM itself.
The internal sensing element or heater circuit can degrade with age and heat, causing the sensor to stop producing a meaningful voltage signal.
Open, shorted, or corroded wires and poor terminal connections can interrupt the signal or reference voltage path, making the PCM see no activity.
Air drawn in through a leak ahead of or at the sensor boss can dilute exhaust gases and prevent the normal voltage switching the PCM expects.
Coolant, oil, silicone, or fuel additives can coat the sensor element over time, slowing or stopping its response even before complete electrical failure.
This is uncommon; the PCM rarely fails to read only one oxygen sensor, and should be considered only after the sensor, wiring, and exhaust have been ruled out.
Sensor 3 is downstream, located after a catalytic converter. On some applications it monitors a second or rear catalyst section.
Usually yes, but the check engine light will stay on and the vehicle may fail an emissions test. Long-term catalyst monitoring may also be disabled.
Almost never. PCM failure causing only one oxygen sensor to read inactive is very rare; replace or repair the sensor, wiring, and exhaust first.
Use a sensor that matches the original application specifications, including the correct connector, wire length, and heated-element type.