I just replaced my GP in the 240D. Resistance was too high across all four, though they would still glow red hot. Resistance across the new ones was normal, so I figured that they were bad but not failed. The light worked fine and the car fired up pretty fast, even at temps as low as 35-40, but the engine would miss and smoke a bit until it got warmed up. New GP seem to have fixed that.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT