If they go bad under the original 4 year 50,000 mile warranty (USA) they will be replaced at no charge. But they are NOT COVERED under the extended warranty from Mercedes-Benz. I just found that out to the tune of $540, with my 10% club discount. The intake manifold has to come off to get to them. On the bright side, they cleaned the EGR gunk from the inside of the manifold while it was off. The glow plugs are listed on Fastlane at 1/2 what the dealer charges, so I'll do them myself next time.

As for why they don't last long, I suspect that it is because the relay keeps them on for a while after the engine is started. Makes for a nice even idle when cold, but probably not so great for the plugs.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References