Problems in a 96 SunFire

Yesterday I took my sunfire for an oilchange afterwards the car runs like crap and they tell me i need a new serp belt and my "map or baro pressure signal is high" so after reading these forums i decided i can do the belt myself. But i dont know anything about the baro pressure signal? can i do this or should i take it somewhere? and how much will it cost???

Did the car "run like crap" prior to the oil change? If not, I'd be hauling my buddy Guido down to the oil change place and asking to see the manager.