Glow plugs heat the combustion chambers of many diesel engines so they can start in cold weather. When they fail, the usual symptoms are hard starting or long cranking when cold, white smoke and rough running for the first minutes after a cold start, misfires that clear up as the engine warms, and a glow plug warning light or code. Failed glow plug relays or control modules cause the same symptoms on every cylinder at once.
Bad glow plug symptoms
- Hard starting or long cranking in cold weather, normal starting when warm
- White smoke and a raw fuel smell after a cold start; see diesel white smoke
- Rough idle or a miss on one cylinder until the engine warms
- Glow plug or wait-to-start light behaving oddly, and glow plug circuit codes
- Higher fuel consumption on cold starts
One plug or the whole system?
| Pattern | Likely cause |
|---|---|
| One cylinder rough when cold | One failed glow plug |
| Every cold start hard, all cylinders | Relay, controller, fuse or wiring |
| Hard starts only below a certain temperature | Weak plugs, weak batteries or fuel gelling |
Testing glow plugs
- Read codes; many trucks report the failed plug’s circuit.
- With the plugs unplugged, measure each plug’s resistance; an open circuit means a failed plug. Compare with the maker’s specification.
- Check that the relay or controller sends power to the plugs when the key is turned on cold.
- Look for damaged connectors and wiring near the valve covers.
Do all diesels use glow plugs?
No. Cummins engines in Ram trucks use an intake air heater (a grid heater) in the intake instead of glow plugs, so hard cold starts there point to the heater, its relays or the batteries; see diesel hard to start when cold.
Glow plug relay and module symptoms
The relay or control module switches high current to all the plugs. When it fails, every cylinder loses its heat at once, so cold starts become hard across the board while each plug tests good. A relay that sticks closed is a different problem: it keeps the plugs powered, which burns them out and drains the batteries. Listen for the relay clicking when the key is turned on cold, and check for battery voltage at the plug bus bar or harness.
Replacing glow plugs
Glow plugs can seize in the head over years. Remove them warm, follow the maker’s procedure and torque, and use anti-seize only where specified. Many owners replace them as a set, because the others are often near the end of their life.
Related
A weak battery also causes slow cold cranking; see how long car batteries last. For no-start diagnosis see diesel won’t start, and for engine replacement see our Cummins engines.
Trouble codes: P0380 Glow Plug Circuit A · P0670 Glow Plug Module Control Circuit · P0671 Cylinder 1 Glow Plug Circuit · P0672 Cylinder 2 Glow Plug Circuit · P0673 Cylinder 3 Glow Plug Circuit · P0674 Cylinder 4 Glow Plug Circuit · P0675 Cylinder 5 Glow Plug Circuit · P0676 Cylinder 6 Glow Plug Circuit · P0677 Cylinder 7 Glow Plug Circuit · P0678 Cylinder 8 Glow Plug Circuit · P0683 Glow Plug Module Communication · all OBD2 codes.
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