P0420 indicates the pre-cat for Bank 1, the firewall side, is the problem. The more expensive of the two, of course.
I too have this problem on my Mystique although in my case it's a DPFE hose coming off which indicates pre-cat is plugging up.
Some relevant info/background.
Late May I had both pre-cats replaced with less than 100 miles remaining under emissions warranty on my '97. Symptom was DPFE hoses were coming off (throws an EGR trouble code). At the time the '98 was past warranty but operating ok. Not so this past Friday; it had a DPFE hose come off and it's now 4k miles past warranty.
Reviewed problem with dealer shop foreman and Service Mgr., advised to call Ford, although they were not optimistic. Did on Tues. and got the party line; no campaign to replace cats past emission warranty mileage but save receipts in case one is announced in the future.
The shop foreman noted that while original recall on 95/96 models called for the three cats to be replaced, under emissions warranty they are now just replacing the manifold cats. However he noted that on my '97 the old front cat appeared ok and in his experience it was the rear manifold that was usually the problem. as it was on mine. Possibly due to front manifold running cooler due to air flow passing thru' rad.
He suggested replacing rear only on my '98, if I ended up having to pay the whole shot. I suggested gutting would also be an option and we agreed to discuss that further.
I'm currently in same quandry as you are: replace rear manifold or gut? Headers, as suggested by others above, is not an option I will be considering.
Welcome to the club!
