qaz wrote:
I suppose it's possible that the P0420 code had disappeared by the time they scanned it. Would be just my luck. I told them I'd be back if it came back (which it probably will in the next couple of days).
  Have them retrieve the Mode $06 test results, and look at TID=$10, CID=$11, and TID=$10, CID=$21.  Perhaps your cats are right on the edge, so that sometimes they pass, and sometimes they fail, the internal diagnostics.  These test results should clearly indicate if this is so.

The question I have is--how can I make the catalyst test run? As I recall there's some special way to drive the car that makes it run all the tests, how do I do that?
  I believe you'll find the drive cycle described in your owner's manual, but a more detailed description can be found at <http://www.obdii.com/drivecycleford.html>.

{I do hate spam, but that isn't really part of my email address.  Remove the string ?HatesSpam? from the email address ?BobHatesSpam@blaylock.to? to email me.  No advertisements.  Spammers will be dealt with very harshly!}
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Moonlight Blue 1996 Mercury Mystique GS , Zetec engine, ATX.  No mods. (Well, OK, one mod.  Before I got rid of my previous car, a 1994 Tempo, I swapped radios.  I liked the Tempo's radio better.  That counts as a mod, doesn't it?  I also kept one of my Tempo's wheels to use as a spare.  I hate those stupid ?compact? spares that come with modern cars.  Does this count as a mod?  If so, I guess I have two mods.  Replaced burned/melted blower switch and connector on 19 February 2002 with a 1P3T toggle switch from Marvac and a Molex connector from Rat Shack; I guess this now makes three mods.)