Electrical System · Expert tip
How to Spot Faulty Diesel Glow Plugs Without Guessing
Stop replacing glow plugs blindly. Learn a systematic test that pinpoints faults using resistance and current draw, so you fix the right problem first time.
The expert tip
- Measure each glow plug's resistance with a multimeter: healthy plugs typically read 0.5 to 2 ohms, but compare to the manufacturer's spec.
- Check current draw with a clamp meter while cranking: each plug should draw 8 to 15 amps initially, dropping as it heats.
- Inspect the glow plug control module for fault codes; many diesel ECUs store specific codes for open or shorted circuits.
- Test the glow plug relay: it should click and supply battery voltage to the plugs for 10 to 20 seconds after key-on, depending on temperature.
- If resistance is infinite or current draw is zero, replace that plug; if all are within spec but starting is hard, check the control module and wiring.
Source: Workshop best practice
Apply tips that match your car and manual. For anything safety-critical or high-voltage, use a qualified mechanic.