MIL Doesnt always mean open loop. if the O2 is reading just out of range the ecm will still use its reading throw a Lean or Rich code and keep running that way. AGAIN get it checked out it could be anything. and besides P0420/P0430 is cat efficiency Not O2 sensor anyway. Wich in itself wont cause an open loop/limp mode either.
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1998 Firebird:buncha parts thats used to make it go fast..... not so much anymore. 
BAAAAAA!!!!!!
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