Help me get my car running right! Exhaust/O2 questions

i just bring that up, cause even with low mile those aftermarket O2's can drive you crazy. i found out the hard way that they are very unreliable.
but you won't believe what i had to pay for Ford replacement O2 for my cobra, but it's a world of difference.
just something to double check, just cause they have only 500 miles that might be the problem.

later
Joe