my wrx sport wagon was showing a 420 code so i took it in to the shop figuring they would replace the cad. converter. right before i took it in some gaskets blue and the car sounded like it didn't have a muffler. when i took it in they replaced the o2 sensor and the gaskets and then stated that while they weren't sure if it would be fixed the check engine light did turn off. it came back on shortly after i got it back from them. this time it showed a p0171 code. they told me that it could be anything from vac. tubes to simply reseting the computer at the dealership. i find it odd that the 420 code had now been replaced by a p0171 code which seems like it could signficantly more complex. any ideas on what happened? im not very educated on these things so any help would be appreciated.