The check engine light went on (97 grand marquis) took the car to advance auto parts they ran the code it came back as catalyst efficiency below threshold bank 1 p0420 The guy said it could be a bad cat or bad 02 sensors.....but he couldn’t give me a straight answer said I would have to take the car into a shop so they could scan it better...I thought that was the point of them having a scanner....anyway does anyone know exactly would this code means......I am thinking of just replacing the 02 sensors and hoping that’s it......what do u guys think.....
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.
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").
