I own a 96 C4 with 130k on it and it likes to throw me a random P0400 code when the CES/SES light comes up.
Sometimes it is fine for 3 or 4 weeks at a time, I clear the code and other times it is once a week with no consistancy to the speed, RPMs, fuel type or otherwise.
My local wrench tells me it is most likely a dirty intake system, and suggested I run some Lucas in the fuel for a while. Now that Im on my 15th tank of fuel and 15th bottle of Lucas, I see no change.
The EGR valve is relatively new, as far as it's shinyness goes. I can also manually press and release the valve by hand with good return. The vaccum solenoid is opening and closing (bench tested) and the lines do not appear to be leaking.
Is there a step Im missing to diagnose this EGR Valve failure code?
Any assistance is appreciated.