On my 1982 240D the glow plug light quits when one plug goes bad. It doesn't start very well when this happens. I just changed them all a few weeks ago after the light stopped coming on and it was getting hard to start.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold