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check engine light

about four months ago i installed a bassani hi-flow x pipe with cats on my 99 mustang gt. i have put on about 3500 miles since then and until two weeks ago had no problems. then the service engine soon light came on. i had the code scanned ( autozone does it for free,while ford wanted $97) it gave a 420 and 430. the codes stand for catalyst effiencey below threshold right side and left. Knowing that 96 and newer cars are very sensitive to exhaust mods, i purchased a set of MIL eliminators, however after installing them the light came back on two days later. the only thing that i can think of that could have caused this was the fact that a couple of days before the light orignally came on, i had filled up and added a can of STP total fuel system cleaner. has anyone else experianced a problem like this? does anyone have a suggestion on how to go about this??

thankx, scott

Re: check engine light

Quote:

Originally posted by scott armstrong
about four months ago i installed a bassani hi-flow x pipe with cats on my 99 mustang gt. i have put on about 3500 miles since then and until two weeks ago had no problems. then the service engine soon light came on. i had the code scanned ( autozone does it for free,while ford wanted $97) it gave a 420 and 430. the codes stand for catalyst effiencey below threshold right side and left. Knowing that 96 and newer cars are very sensitive to exhaust mods, i purchased a set of MIL eliminators, however after installing them the light came back on two days later. the only thing that i can think of that could have caused this was the fact that a couple of days before the light orignally came on, i had filled up and added a can of STP total fuel system cleaner. has anyone else experianced a problem like this? does anyone have a suggestion on how to go about this??

thankx, scott

Scott,

the very basic first:

Did you checked that the MILS were installed in the REAR O2 connetions.
Did you crossed the wires? They are expecific to each O2.
Is that STP O2 and Cat safe?
Did you reset the computer (un-plug batt for about 10 minutes) and installed with the batt diconnected?

Let me know.

Naz