Here's my hypothesis: The results of the first test are pretty close to the results of all the previous tests during my ownership. I haven't gone thru the old paperwork to confirm it, but my recollection is that the truck was never super clean and always relatively close to the limits. During the second test, the tech knew that the truck had just passed the test a half hour before (he had the paperwork in his hand) so rigged the second test so it passed easily. If that's true, then the system can be gamed pretty easily.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13