Testing glow plugs are easy. Follow the wiring from the glow plugs from the front or the engine onto the drivers inner fender. You'll find a plastic cv]over that protects the fuse and plug connector for the glow plugs. If you unplug the connector and turn it over you can measure the resistance from each socket on the connector to ground. You might also replace the glow plug fuse if it doen't look pristine. You are looking for all four glow plugs to be about the same at about 1 ohm each. As someone mentioned above, if in doubt, replace all four, and i'd replace the fuse too. Good Luck, Chuck

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2000 C230 Kompressor "Frosty"

2007 PT Cruiser
2001 VW TDI Beetle
1999 Landrover Discovery II
1971 Corvette

Gone: 97 Cabrio, 83 240D, 93 VW Eurovan, 95 Volvo 850, 93 Chevy Astro Van, 77 VW Bus, 99 VW Jetta, 74 VW Bus