Code P0406: Meaning, Causes, and Solutions
What is Code P0406?
Code P0406 is a generic powertrain code. It is considered “generic” because it applies to all vehicles (1996 models and newer), although repair steps may vary by manufacturer.
This code is related to the EGR system (Exhaust Gas Recirculation), a device designed to reduce combustion temperature and limit nitrogen oxide (NOx) emissions. It does this by reinjecting a portion of the exhaust gases into the cylinders, where they mix with air and fuel. This recirculation slows down combustion and thus lowers the temperature.
The EGR valve can operate in two ways:
- Vacuum-operated: controlled by a vacuum solenoid, which is managed by the PCM (Powertrain Control Module).
- Electronic: directly controlled by the PCM via one or more integrated solenoids.
An EGR valve position sensor allows the PCM to monitor its operation. If the sensor reading is abnormally high for an extended period, code P0406 is stored.
Symptoms of Code P0406
The most common signs include:
✔️ High combustion temperature (causing increased NOx emissions)
✔️ Check Engine Light (MIL) illuminated
✔️ Engine hesitation while driving
✔️ Intermittent stalling
Possible Causes of Code P0406
🔧 Short circuit in the EGR sensor signal circuit to battery voltage (B+)
🔧 Short circuit in the EGR sensor signal circuit to the 5V reference circuit
🔧 Open ground circuit in the EGR sensor
🔧 Open signal circuit in the EGR sensor
🔧 Failure of the EGR sensor or solenoid
🔧 Debris blocking the EGR valve in an open or closed position
How to Repair Code P0406?
1. Basic Checks
✅ Disconnect the EGR valve and try to start the vehicle.
- If the engine does not respond, remove the valve and inspect it for any debris. Clean it and reinstall it.
✅ If the problem persists, block the EGR port and try to restart.
- If the engine starts, the EGR valve is stuck in the open position and must be replaced.
- If the engine still does not start, the issue may be a short circuit in the wiring.
2. Diagnostic Tool Check
✅ Check the EGR position using an OBD2 scanner by comparing the actual value with the desired value.
✅ If the reading is 5V or higher, check the signal circuit for a short to the reference circuit or battery voltage (B+).
✅ Check the ground continuity and repair any detected issues.
3. Diagnosis on a Vacuum-Controlled EGR Valve
✅ If disconnecting the vacuum source allows the engine to start, the EGR solenoid is likely faulty and allowing constant vacuum. In this case, replace the solenoid.
✅ If the engine runs normally despite the code, suspect a wiring problem. Inspect the wiring harness and repair any issues.
✅ If the wiring is in good condition but the code persists, replace the EGR valve.
Other Codes Related to the EGR System
🔎 Related codes: P0400, P0401, P0402, P0403, P0404, P0405, P0407, P0408, P0409