Unscrew the fuel lines and injectors (for reinstallation use a new thermal insulation gasket).
Remove the heat-insulating gasket using a wire hook or a ballpoint pen rod from the injector hole on the cylinder head.
The glow plug leads and wires remain twisted (attached).
Turn the ignition key to the pre-glow position.
Through the holes in the cylinder head (arrow), observe which spark plugs glow weakly or do not glow at all.
It is advisable to replace defective spark plugs; to be on the safe side, replace the entire set.
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