exhaust help!
 

ok so today i got my exhaust done on my car. custom exhaust. its a header back exhaust. 2" header piping, goes into a 3" with a 3" baffled muffler, then splits into a Y pipe with 2 1/4" piping into 2 mufflers. so when i was driving it back it was fine for awhile until my check engine light came on. so i let off the gas and the light went away. then i gave it some gas for a bit and the light came on again. what is happening?? my car is an 89 GXL

hrmm, i don't know too much about s5's but could it possibly be the o2 sensor, or something to do with it?