P0406 is a generic OBDII Diagnostic code. It deals with the signal coming from the EGR system. In this case, it means that the signal coming from the EGR system is higher than the specified range Since it is a generic code, it has the same meaning for the Mercury Mariner as it would any other vehicle.
The EGR system allows a certain amount of the Mariner’s exhaust to re-enter the combustion chamber, which helps lower the emissions coming from your Mariner’s exhaust. When the EGR system has an issue, usually the only thing you’ll notice is the check engine light.
P0406: EGR Sensor “A” → Circuit High
P0406 Symptoms: Mercury Mariner
Since the EGR valve is part of the emissions system, there aren’t always symptoms associated with P0406 (other than the service engine soon light).
Here are the most common symptoms of P0406 when they are present:
- Stalling and/or surging of the engine
- Check engine light
- Higher combustion temp
- Increased emissions output
P0406 Causes: Mercury Mariner
Almost all of the time, P0406 is going to be caused by wiring issues or a bad EGR Valve. We’ll go over both of them in better detail directly below.
Bad EGR Valve
If you happen to have a good scan tool, you can actually use it to command the EGR valve to open. If the EGR valve refuses to open it would be time to check the wiring harness for voltage at the plug that goes into the EGR. If the voltage is OK, then replacing the EGR valve may be a good idea. There’s plenty on testing this wiring in the next section.
Carbon, junk, and even ice can cause the EGR circuit to stick. The most common cause of P0406 is the EGR valve. But, blindly replacing it without testing it can be a waste of money.
If it does end up being the EGR valve, they are pretty affordable.
Wiring Issues
A great place to start diagnosing P0406 in your Mariner is going to be the wiring harness. Being that the wiring harness that goes to the EGR valve is by the hot engine it has a higher propensity to fail than other wiring in the vehicle, it is often the problem.
Find the wiring harness where it starts at the EGR valve. Does the plugin have any visible signs of damage? If it doesn’t, go ahead and inspect the wiring harness for any damage. You’ll be looking for frayed wiring. Look close at any place that it may come into contact with the engine, frame, or anywhere that it takes a sharp turn.
To check the voltage at the harness, you’ll need a volt meter. Check the voltage at the reference circuit. If it’s at 5 volts, but the EGR Valve won’t open, that’s a pretty good indication that the EGR is bad.
If you get 12 volts or 0 volts, that would be an indication of a wiring problem. Here’s a great writeup on diagnosing your wiring issues from AA1Car.com
Conclusion
Most of the time, when P0406 is thrown, it’ll be the EGR valve that is the cause. It is still a good idea to test it and the wiring before buying a new one though.
If there is anything that you would like to add, please leave a comment below. Good luck diagnosing your Mariner!