check engine light on....97 grand marquis

if you know anyone with an Xcal or Livewire, just have them monitor/datalog the O2 sensor. you'll be able to tell by the reading if it is bad.

They guy that scanned it is correct. Its one or the other...

How many miles on the car/cat?

I just had this problem with a '97 Mountaineer. First it started with O2 sensor fault codes, so I replaced the front O2 sensors and crossed my fingers. For about 25 miles, I thought all was well - then the CEL lit up again with the "catalyst efficiency" code - which it was NOT showing before.

I was about to replace the headers and cat-back exhaust parts anyway, so I went ahead and replaced the front cats and the rear O2 sensors; did it all at once.

There's a good chance that fresh O2 sensors (front and rear) will fix this - and if that doesn't do the trick, you're not out any money - the new sensors can be swapped to the replacement cat(s) / downpipe(s). Try your idea first before laying out the cash for new cats (the "EPA tax").