Connect the test light to the battery terminal and connect it to each spark plug in series.
The light is on: the glow plug is OK.
The bulb does not light: defective glow plug, replace. If the glow plug has a burnt-out pin, then it is necessary to strictly follow the operating instructions.
Glow plugs with a burnt pin
Burnt-out glow plug pins are most often the result of faulty injectors. Burnt-out pins cannot cause defects in or on the glow plugs.
If such glow plugs (arrow) are found when making a claim, it is not enough to simply replace them. It is necessary to check the injectors (injection jet, rattling, pressure and tightness) (the work must be carried out in the workshop).
Particular attention should be paid to the appearance of the jet during short, rapid piston strokes (piston stroke in 4-6 seconds) (the stream should be compact and well atomized).
(Read the original source on the website: «AudiManual»)