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How do I know if my glow plugs or grid heater have failed?

Applies to: Cummins, GM 6.2 / 6.5, all diesel engines · Pickups, Vans & cars, Tractors / ag, RV / diesel pushers

CumminsGM 6.2 / 6.5All dieselsPickupsVans & carsTractors / ag+ moreStarting

Short answer

Cold-start aids add heat so a cold diesel lights quickly. Many diesels use a glow plug in each cylinder; Cummins B-series pickups use an intake air grid heater. When they fail, you get long cranking in the cold, rough running and white smoke right after start that clears once the engine is warm.

Safety first. Diesel injection systems run at extremely high pressure; a pinhole leak can inject fuel through skin. Exhaust and turbo parts stay very hot long after shutdown. Never open a hot cooling system. If you are unsure, stop and see a qualified diesel technician.

Start with the easy checks

  1. Watch for a wait-to-start or glow lamp and any warning codes.
  2. Check fuses, relays and connections for corrosion.
  3. Note whether problems happen only when cold.

Likely causes, most common first

Burned-out glow plugs
Tested with the engine cold, unplugged, comparing each plug to the maker's range.
Controller or relay failure
No power to otherwise good plugs or the heater.
Wiring and connections
Corroded or loose high-current connections.

These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.

When to stop and see a diesel tech

  • Removing glow plugs can be risky on some engines (seized or broken plugs); leave it to a shop if unsure.
  • Repeat codes after replacing parts.

Questions

Do Cummins pickups have glow plugs?

The Cummins B-series in Dodge and Ram pickups uses an intake grid heater instead of glow plugs. The GM 6.2/6.5 and many other diesels use glow plugs.

Starting

Why is my Cummins hard to start when it is cold?

CumminsAll dieselsPickupsSemi trucksRV / diesel pushers+ moreTroubleshoot →Smoke

What does white smoke from my diesel mean?

All dieselsPickupsSemi trucksRV / diesel pushers+ moreTroubleshoot →Guides & checklists

How do I get my diesel ready for winter?

All dieselsPickupsSemi trucksRV / diesel pushers+ moreTroubleshoot →

Where this comes from

Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.