In a gasoline engine deposits or discoloration on spark plugs can be used to diagnose problems, for instance lean or rich running, or ignition problems. There are many handy pictures available to show the details of these conditions.

Glow plugs, while they only operate for a short time before starting a diesel engine, still stick out into the pre-combusion or combustion chamber, so are still exposed to the combustion. Can the condition of glow plugs be interpreted for diagnostic purposes?

  • My immediate thought is you can't use them like this. The main reason is because with reading a sparkplug, the sparkplug doesn't heat itself. You rely on the heat from the combustion process to show you what's going on. Every time a glow plug heats up just before startup, it gets hot enough to burn off any deposits left behind. It is basically doing a reset every time they light up, thus not giving you a true indicator. Just my thinking, though. Commented Jun 26, 2023 at 11:32

1 Answer 1


Normally not, people are way too afraid to break them during removal. Since the specialty tool supliers catalogues are littered with "engine xyz glow plug extraction tools" they might have a point.

The only diagnose that gets regularly done is a test for conductivity or resistance.

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