Another interesting glow plug story, like mine where I had a hard to start issue and all glow plugs checked fine and then the next day 2 checked bad.
I think this may be an issue of current hogging... .
I can't explain why 2 of my glow plugs failed at the same time actually, but since it is a parallel circuit, I vote for hogging caused by a cascade .
I just replaced them all and hope they last a few years.
The hogging is common in semiconductors with a low dynamic resistance and can cause this sort of thing to occur without proper safeguards.

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1986 300SDL 201K Daily Driver
1984 300D In Progress
1989 240GL 196K Swedish Brick
1984 300SD -- Sold 289K
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