P2166 indicates that the engine control module (PCM) has detected the throttle pedal position sensor D signal is outside the expected voltage range when the pedal is at its idle or wide-open stop. This typically points to a faulty sensor, wiring issue, or mechanical problem within the pedal assembly, not the PCM itself.
The internal sensor (often one of several in the pedal assembly) has failed, causing an out-of-range voltage at the stop position.
Open, shorted, or corroded wires in the sensor circuit can disrupt the signal, especially at the pedal connector or PCM harness.
A weak ground or unstable 5V reference to the sensor can cause erratic readings at the mechanical stops.
Physical obstruction, debris, or a bent pedal stop can prevent the sensor from reaching its expected idle or WOT voltage.
In very rare cases, the PCM itself may have an internal fault that misinterprets the sensor signal, but this is almost never the root cause.
You may be able to drive, but the vehicle will likely enter reduced power or limp mode, limiting speed and acceleration. It is best to diagnose and repair promptly to avoid unsafe driving conditions.
Often yes, because the sensor is integrated into the pedal assembly and not serviceable separately. However, always verify with proper diagnosis first, as wiring or connector issues can mimic a bad sensor.
Almost never. The PCM is rarely at fault for this code. Most cases are resolved by repairing the sensor, wiring, or connector. Replacing the PCM without proper diagnosis is a common and costly mistake.
If the fault is intermittent and the condition no longer exists, the code may clear after several drive cycles. However, it is recommended to clear the code with a scan tool after repair to confirm the fix.