Hi Joker,
I had this code too and I was OVER the 8/80,000. The problem stemmed from a bad misfire on two cylinders that caused 'neat gas' to go out the exhaust and 'partially contaminate' one of the cats to the point that the 'efficiency' dropped and so the CEL went on.
The fix was to, of course, stick in some new HEI-wires and plugs (if needed) AND...
...to 'clear the cats' !!!!
Read: you need to stick in a can of 'Catalytic Converter Cleaner' into your gas tank AND/OR a can of 'Berryman's B12' or 'seafoam'.
You'll need to do this for, probably, 3 or 4 fill-ups before the CEL completely stops.
This is a 'progressive' (albeit cheap) fix.
Worked for me ...now going on TWO years since I got the 420 code ...(I swear by Berryman's B12!) -- and was a lot 'cheaper' of a fix than going the $tealership route. (They quoted $2,500 to replace the bad cats -- God don't you just HATE dealing with $tealerships sometimes!!!)
Latah,
G.
Oh yeah, btw, splash out for a code scanner -- try ebay -- it'll 'save' you from having to pull fuses or disconnect the battery every time the CEL comes on.
Also, do NOT drive with the CEL on! ...the car goes into "limp home mode" and ENRICHENS the mixture -- further exacerbating the problem of 'semi-clogged-cats'!
