Quick answer: The P0406 code indicates that the Powertrain Control Module has detected an abnormally high voltage signal from the Exhaust Gas Recirculation Sensor A circuit. This affects the emissions control system and requires moderate urgency. You should address it soon to avoid failing a state emissions inspection or reducing your fuel efficiency.
Seeing a Check Engine light can ruin your day, especially when an OBD-II scanner returns a specific powertrain definition like P0406. This guide breaks down what triggers this fault, how to diagnose it without wasting money, and what you can expect to pay for a lasting repair.
What Does P0406 Mean?
Your vehicle’s Exhaust Gas Recirculation system helps lower combustion temperatures and reduce harmful nitrogen oxide emissions by routing a measured amount of exhaust back into the intake manifold. The EGR valve is equipped with a feedback sensor-often called Sensor A-which tells the computer exactly how far the valve is opening. Code P0406 trips when the voltage signal from this sensor stays higher than the normal operating range, usually above 4.8 or 4.9 volts, for too long. The computer sees this as a frozen, shorted, or malfunctioning sensor circuit.
Symptoms
- Check Engine light illuminated on the dashboard
- Engine hesitation or rough idle, particularly when coming to a stop
- Engine pinging or knocking under acceleration due to excessive combustion chamber temperatures
- Slight decrease in fuel economy
- Failure during a local emissions or smog test
Possible Causes
Before you spend money, keep in mind that the most common mistake is replacing the entire EGR valve assembly when the real issue is just a dirty connector or a broken wire.
- Corroded, loose, or damaged wiring harness connector at the EGR sensor
- Open or short in the signal wire running between the EGR sensor and the powertrain computer
- Carbon buildup holding the EGR valve pintle open or jamming the sensor mechanism
- Failed EGR position sensor itself
- Internal failure of the Powertrain Control Module, though this is rare
Check These Yourself Before Visiting a Mechanic
- Locate the EGR valve on your engine, unplug its electrical connector, and look for corrosion, bent pins, or trapped moisture.
- Inspect the visible wiring harness leading away from the EGR valve for any chafing against hot engine components or metal brackets.
How to Fix It
- Clean any corrosion out of the EGR electrical connector with electronic contact cleaner and secure the locking tab.
- Use a digital multimeter to check the reference, ground, and signal voltage lines for continuity and short-to-power conditions against wiring diagrams.
- Remove the EGR valve and inspect the pintle and internal passages for heavy carbon deposits that might prevent proper movement, cleaning it with dedicated solvent if necessary.
- Replace the EGR valve assembly if the internal position sensor has failed electrically or mechanically.
Estimated Repair Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Wiring Repair / Terminal Cleaning | 10 – 30 | 50 – 150 |
| EGR Valve Cleaning | 15 – 40 | 100 – 200 |
| EGR Valve Replacement | 120 – 350 | 100 – 250 |
Note: these are 2026 estimates in US dollars; costs vary by make and model.
Frequently Asked Questions
Can I drive with the P0406 code?
Yes, your car will usually still drive normally, but your emissions will be dirtier and your fuel economy may suffer slightly. You should get it fixed before your next mandatory vehicle inspection.
How do I clear the P0406 code?
You can clear the code using an OBD-II scanner tool plugged into your dashboard port. However, if the underlying electrical short or mechanical sensor fault is not repaired, the Check Engine light will return quickly.
Is the EGR valve the same thing as the EGR sensor?
Not always. On many modern vehicles, the position sensor is built directly into the EGR valve assembly as a single unit, meaning you must replace the entire valve if the sensor fails. On others, it is a separate replaceable component.