itsa 96 hatch dx auto car runs perfect but when its warmered up an im sittin in traffic the idle goes really high an doesnt ever come back down i got the engine light checked at auto zone an they said it could b the cat but i think they full of **** an want me to pya the 600 that they charge ne one have ne ideas i was figurin o2 sensor or iacv or somthin lemme kno thanks

 

what code was it throwing?

 

there scanner said p0420 or somthin like that an it said check catlytic converter an a misfiring which its not an vacumm leak which it doesnt have an burnin oil which it doesnt do

 

this is just a idea. Um check the upstream o2 sensor. An by check I just mean swap.