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Q:

o2 sensor on 1998 Ford Mustang

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
check engine light is on and has custom exhaust, can that be it?
(2) Answers
It could be the CAT if it was removed i would get a diagnostic to see what the problem is autozne does this for free.
I had the same promble (check engine light). I stop using cheap gas and after 3 tanks of gas (plus), the light went out.