DT wrote:
My question: was the car idling at 2000 RPM when P0402 came on? Mine idles at about 650 RPMs.
  No.  I do not think I've ever seen my engine idle that high, othe r than immediately after a cold start.  When I've been able to correlate the P0402 code with engine activity, it's matched up with a drop in idle speed and a marked amount of roughness in the idle.

  I believe the test has been discussed in this thread, whereby you apply a vacuum to the EGR valve while the engine is idling.  If all is well, doing this will cause the engine to either stall or to idle very, very badly.  The effect that I have experienced that matches up with the occurrence of the P0402 code (and which I often get without generating a code) is much like the bad idle you get if this test doesn't quite kill the engine entirely, but not quite that bad.

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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.)