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Word of caution - EGR valves and Exhaust Systems get very hot.
What is an EGR valve for heavens sake? EGR stands for "Exhaust Gas Recirculation. Some of your exhaust is fed back to the engine through a valve (usually vacuum operated) to lower combuston temperatures and reduce NOX pollutants. As your engine wears, horrible black grunge builds up in the EGR and closes it off like an artery. The net result is .. you fail the emissions tests - Wonderful. Mechanics love them. Why? Because they can clean them out, shine them up and charge the living heck out of you for "emissions repair" Quite a little cottage industry are EGR Repairs.
What is it?
"Definition: The EGR valve is the main emissions control component in the exhaust gas recirculation system. The valve is located on the intake manifold and meters a small amount of exhaust gases back into the intake to lower combustion temperatures....." Does not get any more boring than that.
So, as the main part of the vehicle emissions system, the EGR reduces combustion temperature to control oxides of Nitrogen ( NOx) being released.
When it fails it ... Causes Pinging or engine knock when the engine is under load.
A code P0401 means ....
There is a blockage in the tube going to the EGR, or from the EGR to the intake (carbon buildup) from worn engine. Maybe the EGR valve is bad - won't open perhaps due to a lack of vacuum
The EGR valve is easy to find. Follow the exhaust from the exhaust manifolds and you will find a tap off them near the engine. Follow this to a valve that seems connected to the intake - The EGR valve. Replacing the EGR may NOT stop the code from returning. You should test the EGR valve first. Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's. Yes, you can suck it open with your mouth! Horrible, but doable! A working EGR will cause a drop in RPM's or even kill the engine when you force the EGR open at idle.(it's only supposed to open later on in the rev range) Clean out the EGR valve and the tubes leading to, and from the valve (if any) to remove deposits I like to hit them gently (ok hard) with a hammer - not very efficient but fun as heck. All the black stuff comes pouring out. B12 will sometimes clean it too, but not as much fun.