Bad glow plug symptoms and how to test them
Glow plugs heat the combustion chamber so a cold diesel can light its fuel. When one or more fails, the first thing you notice is a rough, smoky cold start. They are cheap parts that can become expensive jobs if you are unlucky removing them.
Symptoms
- Hard starting or no start below about 40 F.
- White or grey smoke for the first minute or two after a cold start.
- Rough, clattery idle when cold that smooths out as the engine warms.
- Glow plug light flashing or staying on, or a check engine light with glow plug codes (P0380, P0670, P0671-P0678).
- Misfire codes on cold start only.
Which engines use glow plugs?
Duramax, 6.7 Powerstroke, older Powerstroke engines, most European and Asian diesels, and most light diesels use glow plugs. Cummins pickup engines (5.9 and 6.7) use an intake grid heater instead, which warms the intake air. A failing grid heater or its relays gives the same cold-start smoke. See 6.7 Cummins problems.
How to test glow plugs
- Scan for codes first. Many engines identify the cylinder.
- Disconnect the battery, then disconnect the wire or connector from each glow plug.
- Measure resistance from the glow plug terminal to engine ground with a multimeter. A good plug is usually under about 2 ohms (often 0.5-1.5). An open circuit (OL) means the plug is dead. Check your engine's spec.
- Check the glow plug harness and connectors. Corroded or burned connectors are common on some engines and cause codes with good plugs.
- If all plugs test good, check the glow plug control module or relay, and its fuse. On many trucks the module is the actual failure.
- An amp clamp on the supply while glowing is the quickest test: each plug draws a known current, so a low total points to a dead plug.
Removing them without snapping one
Glow plugs seize in aluminum heads, and a snapped plug can mean pulling the head or paying for a special extraction kit. To reduce the risk:
- Do the job with the engine warm, not hot.
- Soak them with penetrating oil for a day if you can.
- Use a quality deep socket and steady force. If it will not move, stop, warm it, and work it back and forth gently.
- Some engines have specific tool kits and procedures; follow them.
- Use anti-seize only if the manufacturer allows it; some plugs are coated and must go in dry.
- Torque new plugs to spec. Overtightening damages the plug and the head.
Typical cost
| Job | Typical range |
|---|---|
| One glow plug | $150 - $400 |
| Full set (V8 diesel) | $400 - $1,000 |
| Glow plug control module | $150 - $500 |
| Snapped plug extraction | $300 - $1,500+ |
Related trouble codes
FAQ
Can a diesel start with bad glow plugs?
In warm weather, usually yes. In cold weather a diesel with several dead glow plugs may start very hard or not at all, and will smoke and run rough until warm.
How long do glow plugs last?
Many last 100,000 miles or more, but it depends on the engine and climate. Constant short trips and cold starts wear them faster.
Should I replace all glow plugs at once?
If one has failed at high mileage, others are likely close. Many owners replace the full set to save labor, but it is not mandatory.
Do Cummins engines have glow plugs?
Cummins 5.9L and 6.7L pickup engines use an intake air grid heater instead of glow plugs.
More diesel guides
General guidance only. Check the service information, recalls and bulletins for your exact vehicle. Costs are rough 2026 US estimates, not quotes. Need a number for your truck? Try the repair cost estimator.