P0406 indicates that the Powertrain Control Module (PCM) has detected a high voltage condition in the EGR (Exhaust Gas Recirculation) sensor circuit, meaning the sensor is reporting a voltage above its normal range.
The sensor itself has failed and is sending an abnormally high voltage signal to the PCM.
Damaged wires, corrosion, or a short to voltage can cause the PCM to read a high voltage condition.
A loose, dirty, or corroded connector can create an intermittent high‑voltage reading.
When the valve cannot close properly, the sensor may be forced into an out‑of‑range position, triggering a high reading.
A rare internal failure in the PCM can misinterpret the sensor signal, but this is far less common than sensor or wiring problems.
No. In the overwhelming majority of cases the problem is the sensor, wiring, or connector. Replacing the PCM is rarely needed and is usually not the correct repair.
Yes, a mechanically stuck or failed EGR valve can force the sensor out of its normal range, resulting in a high‑voltage reading.
The vehicle will operate, but you may experience reduced performance, higher fuel consumption, and increased emissions. It is advisable to diagnose and repair the issue promptly.
Wiring issues are a common cause—especially in older vehicles where harnesses can become brittle or corroded—so checking the circuit should be an early step.