The 6.7L Cummins does not use glow plugs - it warms intake air for cold starts with an electric grid heater in the intake. When that grid heater stops cycling, cold starts get hard, the engine stumbles and puffs white smoke until it warms, and you may notice the lights no longer dim the way they used to on a cold morning. Here is how to diagnose a grid heater that will not cycle.
Common causes on the 6.7L Cummins
- Failed grid heater relay(s) - the high-current relays that switch the grid element are a common failure and stop the heater from energizing.
- Burned or failed grid heater element - the intake heater element itself can fail open and stop drawing current.
- Grid heater bolt failure - a well-known 6.7L concern where the grid heater bolt can loosen or break; besides no heating, a dropped bolt can cause serious engine damage, so this is worth inspecting.
- Wiring, connector, or ground faults in the high-current circuit.
- ECM not commanding it - a temperature sensor fault can leave the ECM thinking it is warm enough to skip the heater.
How to diagnose it
- On a cold start, watch for the wait-to-start behavior and a voltage/light dip as the grid draws current; no dip suggests the heater is not energizing.
- Measure current or voltage at the grid heater during the cold-start cycle to confirm it is switching on.
- Test the relays and check for command from the ECM with a scan tool.
- Inspect the grid heater bolt and connections - a loosening bolt is both a no-heat cause and a serious risk on the 6.7L.
- Scan for intake-heater and temperature-sensor codes.
Fixing it
Replace failed relays and repair wiring or grounds in the high-current circuit. If the element is failed, the grid heater is replaced. Take the grid heater bolt issue seriously - inspect it and address it per the known 6.7L service guidance, because a dropped bolt can wreck the engine. Fix any temperature-sensor fault so the ECM commands the heater when it should. Remember this is a Cummins grid-heater system - it has no glow plugs, so do not diagnose it like a Power Stroke or Duramax cold-start system.
Frequently asked questions
Does the 6.7L Cummins have glow plugs?
No. The 6.7L Cummins uses an intake grid heater to warm incoming air for cold starts, not glow plugs. Cold-start hard-start diagnosis focuses on the grid heater, relays, and wiring.
How do I know the grid heater is working?
On a cold start it draws heavy current, which usually causes a brief voltage or light dip. No dip, plus hard cold starting and white smoke, suggests the heater or its relays are not cycling.
Is the grid heater bolt really dangerous?
It can be. The 6.7L grid heater bolt is known to loosen or break, and a dropped bolt can cause major internal engine damage. Inspect it when diagnosing grid heater problems.
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