Code P0400 need help.
 

My sons 96 Z28 has the SES coming on and always p0400. We replaced the vacuum hoses anywhere they looked dry. Then the EGR valve and now the control solenoid for the EGR. It seemed OK yesterday, got through a cycle where all said ready or ok for all monitors, we got the car inspected so at least that is done, but the code came back this morning. The car has Flowmaster 3in exhaust, Random Tech Y-pipe, 52mm TB and 1.6 rocker arms on it from stock. Car has had all these things for 3+ years with the last being the rocker arms while changing the lifters in April 07 and has not had the code before just after inspection last year, but the car runs perfectly so we only tried to fix because of inspection. Any ideas on what to look for next, any help would be appreciated.

Check the corrugated EGR riser pipe that connects from the passenger side exhaust manifold to the back of the intake manifold. Could be cracked or leaking.

The actul diagnostic test is only run once per trip, and the operating conditions have to be very stable, like steady speed cruise with a constant throttle position and constant MAP for 1.5 seconds preceeding the diagnostic test.