Engine cranks normally but won't fire, especially in cold weather (below 15C). Glow plugs heat the combustion chamber to help ignite diesel fuel during cold starts. If they're burned out, cold starting becomes impossible.

What you'll see

The engine cranks at a good speed -- the battery and starter are fine -- but it just won't catch. You might get a brief cough or puff of white smoke, but it won't run. This happens mainly in cold conditions. In warm weather (above 20C), the engine might start OK even with bad glow plugs because the ambient heat is enough. The glow plug indicator light on the panel may not illuminate, or may illuminate but the plugs aren't actually heating.
Before you assume this is the problem

If the engine cranks slowly, the issue is the battery or starter, not the glow plugs. If the engine starts and runs for a few seconds then dies, that's more likely a fuel or safety shutdown problem. See: Weak or Dead Battery, Engine Starts Then Shuts Down.

See all causes of engine won't start →

How to diagnose

  1. Check the glow plug indicator light

    Turn the key to the 'on' position (or press the preheat button). The glow plug light should illuminate for 5-30 seconds depending on ambient temperature. If the light doesn't come on, the glow plug relay or timer may be faulty. If it does come on but the engine still won't start, individual plugs may be burned out.
    Result: Light comes on and stays for appropriate preheat time.

  2. Test individual glow plugs

    Disconnect the power wire from each glow plug. With a multimeter set to resistance (ohms), measure between the glow plug terminal and the engine block (ground). A good glow plug typically reads 0.5-2 ohms. An open circuit (infinite resistance) means the plug is burned out. You can also test them with a 12V supply -- a good plug should glow red-hot at the tip within a few seconds.
    Result: 0.5-2 ohms = good. Infinite/open = burned out. Even one bad plug can prevent cold starting on a 3 or 4 cylinder engine.

  3. Check the glow plug relay and wiring

    The glow plug relay supplies high current to all glow plugs simultaneously. Check for power at the relay output when the preheat cycle is active. If no power at the output but the indicator light works, the relay is faulty. Also check the heavy wire from the relay to the glow plug bus bar for corrosion or loose connections.
    Result: Battery voltage at relay output during preheat = relay OK. No voltage = relay faulty.

How to fix it

  1. Replace burned-out glow plugs

    Replace any glow plugs that test open-circuit. They unscrew from the cylinder head like spark plugs. Apply anti-seize compound to the threads to prevent them from seizing in the head (they operate in extreme heat). Torque to spec -- don't over-tighten. Replace all of them at once if the engine has high hours, because if one failed the others are likely close behind.

  2. Replace the glow plug relay if faulty

    The relay is usually mounted in the engine compartment near the battery. It's a heavy-duty relay that handles 40-80 amps. Replace with an exact match. Check the timer circuit or controller that drives the relay -- the preheat duration should extend in colder temperatures.

Common mistakes

Don't skip the preheat cycle. Wait for the glow plug light to go out before cranking. Some operators impatiently crank immediately -- on a cold day, this just wastes battery. Also: some diesel engines use inlet air heaters instead of glow plugs (common on larger engines with Deutz, Caterpillar, or Cummins). These heat the incoming air rather than individual cylinders. Same symptom, different component -- check your engine manual.

Parts & tools

Multimeter. Glow plug socket (deep socket, usually 10mm or 12mm). Anti-seize compound. Replacement glow plugs (match engine model). Replacement relay if needed.

Review safety precautions before starting →

Safety

Glow plugs get extremely hot -- over 1,000C at the tip. Never touch them immediately after testing. Allow cooling time before handling.

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