That still doesn't explain the sluggish starting it must be all glowplugs failed.
Correct - I was just replying to your previous statement Captain

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The glow plug light going out merely means that the engine is ready to start - it does not mean that the glow plug control circuit has switched off.
As has been previously posted by others - it is easy to individually test the glow plugs for resistance
in situ (usually approx 1.0 - 1.2 Ohms) and even easier to test for power from the glow plug control circuit - no need for any fancy test gear or guesswork.
Diesels do need a very good battery for a successful start on a cold morning - it takes a lot of elec energy to spin the engine over whilst the glow plugs are 'on'.