I have mentioned this several times, but it might be worth mentioning again. When doing the glow plug resistance test, do a before and after glow test. That way you'll know for sure they're all working. The resistance should be much higher when they're hot (on my meter it shows as a high negative number that's dropping).

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL