loopcycle

When we first took the car in to have the CEL diagnosed, the dealer reported code P0442 and recommended that we have the catalytic convertor and o2 sensors replaced, which we did to about the tune of $1300. Now, the CEL is back on again about two months later, and I'm seeing diagnostic code P0442 again (thank you Autozone). It does not appear to be a gas cap problem as ive torqued it down several times and still see the CEL. The dealer says that if they find problems with their repairs they will handle everything free. But, if they find nothing wrong with their work, they will bill me a diagnosis fee of $95, on top of any additional repair work that I elect for. Whether its relevant or not, I got the impression they dont take diagnostic codes from outside their repair shop. Maybe the diagnostic fee includes other tests or an overall inspection. I sense a conflict of interest here. I'm almost tempted to take the car to another dealership, but the dealer that did the repair work has a 12 month/12000 mile warranty. Any thoughts on this? Is there anything obvious that I'm missing, or any relatively simple things I can check out myself? I searched the forums but found very little information on evaporative emission leaks that would cause this. TIA.