Years. How many I don't know, but I had two bad ones on the Volvo when I did the injection pump about 4 years ago, and had to replace two again two years later, but I think that was because the relay was bad and the glow plugs stayed on all the time -- ran the battery flat on the road once, draws so much current that the alternator can't keep up with the headlights on!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!