Cold Starting · Expert tip
Diesel Cold Start Problems: Glow Plug Symptoms to Check
Hard cold starts in a diesel often point to glow plugs or their control circuit. Here is how to test them and spot the symptoms before they leave you stranded.
The expert tip
- Count the glow plug light: if it stays on longer than 10 seconds at 0 °C, the control module is extending the afterglow because one or more plugs are weak.
- Measure glow plug resistance with a multimeter: a healthy plug reads 0.5 to 2 ohms; anything above 5 ohms or open circuit means replace it.
- Check for voltage at the glow plug harness during the first 5 seconds of key-on; if 11 to 12 volts is missing, test the relay and fuse.
- Look for white smoke and a rough idle for 30 to 60 seconds after a cold start, a classic sign that one cylinder is not firing until heat builds.
- Remove and inspect the glow plug tip: heavy carbon or a swollen tip indicates the plug is failing and may be hard to extract.
- Replace glow plugs as a set if one fails and the engine has over 100,000 km, because the rest are likely worn equally.
Source: Workshop best practice
Apply tips that match your car and manual. For anything safety-critical or high-voltage, use a qualified mechanic.