Learning how to test glow plugs saves you from guessing on cold-start problems. Glow plugs warm the cylinders so a diesel lights off cleanly; when they fail you get hard cold starts and white smoke. Here is how to check them.
What glow plugs do
On glow-plug engines like the 7.3L and 6.0L Power Stroke, each glow plug is a heating element that warms its cylinder for cold starts. A controller (relay/module) powers them for a timed cycle. (Note: Cummins engines use a grid heater instead of glow plugs.)
Testing the plugs
With the plugs accessible, a resistance check across each plug tells you if it is open (bad) or within range (good). An open or out-of-range plug is replaced. Compare all plugs - a mix of good and bad explains a rough cold start.
Testing the controller and wiring
Confirm the glow plug relay/controller sends power for the correct cycle, and inspect the wiring and connectors that feed the plugs. A control-circuit fault (e.g. code P0670 on the 6.0L) points here rather than the plugs.
Reading the symptoms
- Hard cold starts and extended cranking
- White smoke and rough running until warm
- Little trouble once the engine is warm
Frequently asked questions
How do I know if a glow plug is bad?
A resistance check shows an open or out-of-range plug. Symptom-wise, hard cold starts with white smoke point to glow plugs.
Do Cummins engines have glow plugs?
No - Cummins uses a grid heater. This test applies to glow-plug engines like the Power Stroke.
Testing glow plugs is simple: check resistance on each plug and confirm the controller. Replace the failed plugs and cold starts come right back.
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