P0401= Exhaust Gas Recirculation Flow Insufficient Detected
First verify your EGR plumbing is free and clear. As you spray clean the EGR ports, make sure you can see the liquid dump out the end of the tube behind the throttle plate. If that's good, it's got to be the EGR valve itself, or maybe it's wiring. There is a write-up showing how you can hack the (1998+)EGR vlv apart and fix the 'feedback' portion of it yourself.
G/luck
Joel