The first frosty mornings of October are when we see the same question again and again: "My diesel started fine all summer, now it cranks for ages and smokes white." In most cases the culprit is the pre-heating system – the glow plugs – not the starter or the fuel.
How glow plugs work
A diesel has no spark plugs. It ignites fuel by compression heat, and on a cold engine that isn't enough. Glow plugs heat the combustion chambers for a few seconds before and after starting. If one or two of them have failed, the engine still starts in summer, but in the cold it struggles.
Typical signs
- Long cranking on cold mornings, starts easily once warm.
- Rough running and white or blue-grey smoke for the first minute.
- The coil-shaped glow plug light goes out immediately, or flashes while driving.
- Fault codes such as P0380 (glow plug circuit), P0670 (glow plug control module), P0671–P0674 (a specific cylinder) or P0683 (communication with the control module).
What we check, in this order
- Battery first. A weak battery drops voltage while the glow plugs draw current, and the starter then turns too slowly. Rest voltage below about 12.4 V is a warning sign; P0562 (system voltage low) often appears at the same time.
- Each glow plug's resistance with a multimeter – a good one usually reads under 2 ohms, a dead one shows an open circuit.
- The glow plug relay or control module and its wiring.
- If the plugs are fine: fuel filter (paraffin wax in summer diesel can block it in frost) and injector leak-off.
Practical tips for winter
- Wait until the glow plug light goes out before cranking. On many cars, turning the key to "on" twice gives extra pre-heat.
- Replace glow plugs as a set if several are old – they tend to fail one after another.
- Removing seized plugs on some engines needs care: a snapped plug can mean taking the cylinder head off. We soak and warm them up before trying.
- Switch to winter-grade diesel early and change the fuel filter if it is due.
Our assessments combine AI analysis with real workshop experience: you can enter the fault code or describe the symptoms in Diagno's diagnosis on https://diagno.ee/en/ and see the likely causes in a minute.
Fault code pages for reference
- P0380: https://diagno.ee/en/code/P0380
- P0670: https://diagno.ee/en/code/P0670
- P0671: https://diagno.ee/en/code/P0671
- P0683: https://diagno.ee/en/code/P0683
- P0562: https://diagno.ee/en/code/P0562
If you'd like us to measure the glow plugs, battery and charging system properly, you can book a diagnostic check here: https://diagno.ee/en/#broneeri
Over to you: how is your car coping with the first cold mornings this year – any slow starts or warning lights yet?