I put in 5 new ones two years ago, and had one go bad this year. Also, aside from total failure, glow plugs lose their ability to heat very fast and very hot over time. Being in Massachusetts, with cold winters, I put in new battery and new glow plugs every three years. If I do this, I can usually start the car even when outdoor temps are in the zero F range....maybe even at 10 below (???). I routinely use a block heater, but if I go up to New Hampshire in the mountains for an overnight, and wake up to 10 below, I will have real problems if my battery or glow plugs are marginal. (My car is 85 300D, pencil type glow plugs)

Mark