I had three glow plugs go bad after about a year. I tested them from the terminal and then tested them right at the plugs. I got a reading of infinity on 4 and 5 and a reading of something other that 1 on numbe 3. After talking with a trusted MB tech in the area he told me that there was a bad batch of, if I remember correctly, Monark Diesel glow plugs produced last year. This was news to me. Anyway, I bought 5 new Bosch plugs and installed them. No problems after that. I must note too, that I couldn't understand why my car wouldn't start even with the "new" plugs that I installed last year. My tech suggested testing the plugs and thats when I found the bad ones. I've learned now, if there is a no start situation, always check the plugs first. It will save you headachs.