I have 2004 Quest - only 30K miles. Had Service engine soon light come on (P0403). I took it to nearby NTB, hoping to pay less than the dealership. They replaced the EGR valve (about $630). The light was still on. They suggest that I go to the dealer. The dealer went through diagnostics ($100), quoting "checked for catalyst debris - nothing found, checked power to EGR, reset MIL, cleared self learning & performed idle volume learn procedure". The light is still on. They thought the part may have been bad and pointed me back to NTB. NTB replaced the valve with another one, but the light is still on, so they gave up on me and said they can't do anything. Should I be piling money into more diagnostics? What's the best course of action? I'm frustrated because few months back I just spent hundreds of dollars fixing other things (throttle chamber assembly, camshaft and crankshaft positioning sensors) - all this just as soon as my car ran out of warranty.