With my 300D, I got normal glow plug light operation with one bad plug (#3 I think) and only noticed something was up when my car was hard to start one cold morning in Grand Junction. When I bought the car, I had two bad plugs (don't remember which ones), and the light would come on briefly and then go out. The car started with difficulty when cold. Since I got the car out of storage last month, the glow plug light doesn't come on at all. I vaguely remember a thread somewhere that said if the #1 plug, and only the #1 plug is out, that the light won't come on. The car starts normally in warmer weather, and with just enough hesitation in cooler weather to make me believe I may have one glow plug out. However, the car has other issues (vacuum, no dash lights, blower works intermittently) and I am lazy, so I haven't checked it out.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.