Q: Check Engine Light keeps coming on... on 2004 Chrysler PT Cruiser

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In March I had my water pump, serpentine belts, seals, and timing chain replaced. Since then, my check engine light comes on the third start after I get it back from the dealer/shop - every time. I've had it in at least six times since the work in March - had sensors replaced, wiring fixed, etc. Any ideas - the dealership is running out of pssibilities.
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I don't know what code is coming up - it is the same code every time though. The dealer has been very good about all the work and have not charged me for anything they've done with regard to trying to repair this. They reprogrammed the sensor - one of the emissions sensors that was on a recall notice - thesensor has been replaced per the recall - and we thought that would fix the problem - but it didn't. They did find a problem with the multi-function switch and ordered a new one. As soon as they get it in, they will replace it. The dealership's mechanic seems to think it's an emissions problem according to the code - whatever it may be - we just haven't figured out what problem. It didn't start acting up until they replaced the water pump, seals, and all that stuff in April.