Re: Trying to figure this out...
how many miles on the truck? Do you feel a reduction in power at all? A clogged cat will cause poor performance and gas mileage, in OBDII cars (1996+) a clogged cat will trip the downstream 02 sensor and trigger a CEL. Even so, a 1990 should have a downstream 02 sensor after the cat. It could be a clogged cat (seems more likely) or a bad 02 sensor. Not sure how pre OBDII computers functioned, but even a bad 02 sensor (meaning the cat is perfectly fine) will cause the car to operate in a fail safe mode, where the computer makes the engine run rich (hence the poor gas mileage).
2006 Fx4 Level 2 with a 5spd, Superlifted on 33's.