Why not just have AutoZone or one of the other McParts stores read the code for you (for free)? They are less likely to reset it for you these days, but you can at least find out what's causing the CEL.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)