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?
I had a high mileage Blazer that would throw a sporadic EGR code as you describe. It was caused by chunks of carbon getting stuck in the EGR pintle/orifice. Found ~1/16 to 1/4" sized chunks several times.
Cleaning it every few weeks was a pain - even more often if a long trip was involved. Tried blocking off the EGR with a piece of sheet metal and it threw a code instantly. A piece of stainless screen wire greatly extended the cleaning cycle but didn't eliminate the need for it. I suppose a new set of valve seals or guides or rings was really needed.
If you remove the EGR for inspection, look for carbon deposits.
If you really think it's a dirty intake, you can try some Seafoam to clean it. Do a search on it on this forum. Good stuff.
I had the codes P0400 on my '96 LT1 with 62,000 miles recently and just replaced the EGR valve as well as the solenoid valve. Seems to work fine now. ... however, when I had the fuel rail cover off, I noticed a leaking fuel pressure regulator at idle! I replaced this also. Whew, I was thankful I noticed it!