Connect ohmmeter (see illus.) and measure passage resistance. Passage resistance of sheathed glow plug at normal temperature is approx. 0.4 ohms.
Inspect
After removal, connect sheathed glow plugs to 12 V source. For this, connect one cable to plug casing, the other to plug connection thread.
If sheathed glow plug is in good condition, the glow plug begins to glow at front. The glow test must occur briefly (until the glow is visible), otherwise the glow plug can be damaged.