The 4-1-6 code was for bad boost pressure reduction signal from the tranny control module. Are you the original owner? Just wondering if a previous owner did some tinkering with the waste-gate on the turbo (too aggressive adjustment on the waste-gate arm...). The TCM sent a signal to the ECU to backoff on the boost pressure and it didn't (or couldn't) and set the DTC fault (416). Anyway, the 4-3-6 code is just the rear O2 sensor and you just replace it with a new one. FCPGroton.com has them for $108 if I remember right. They are heated units and sometimes the heating element craps out and the computer logs a DTC as a bad sensor even though the motor seems to run fine. I would just replace it. Good luck .......... Mike