Glow plugs preheat the combustion chamber so a diesel fires cleanly when cold. They are found on Power Stroke and Duramax engines (and many light-duty diesels). Important fitment note: Cummins engines do not use glow plugs, they use an intake grid heater instead, so this guide applies to glow-plug engines. Hard cold starts, white smoke on startup, and a rough cold idle often trace to dead glow plugs or a failed controller. Here is how to test both.
How the system works
A glow plug control module (GPCM) powers the plugs for a set time based on temperature. Each plug is a resistive heater that should show low resistance when good. A single dead plug can cause a cold miss; multiple dead plugs or a bad controller cause hard starts and heavy startup smoke.
Step-by-step testing
- With the engine cold and off, disconnect each glow plug's electrical feed.
- Set a multimeter to ohms and measure resistance from the glow plug terminal to engine ground. A good plug typically reads a low value, often under a few ohms; an open (infinite) reading means the plug is dead.
- Record each cylinder so you know which plugs to replace.
- Test the controller: with a scan tool, command the glow plug relay on and confirm it activates, or back-probe the controller output for voltage during the glow cycle with the key on.
- Check the feed to each plug during the glow cycle, no voltage at a plug with a good controller output points to wiring or the harness.
- Inspect for physical damage, a swollen or tip-eroded plug should be replaced and can even break off during removal, so heat-soak and penetrant help.
Reading the results
Replace any plug reading open or far out of line with the others. If all plugs test good but none get power, suspect the controller, relay, or wiring. Because glow plugs can seize in aluminum heads, warm the engine slightly, use penetrating oil, and back them out carefully to avoid breakage.
Removing seized glow plugs without breaking them
The scariest part of a glow plug job is not the electrical test, it is getting an old plug out of an aluminum head without snapping the tip off inside the cylinder. Carbon builds up around the tip and the dissimilar metals seize together over the years. Rushing this step turns an afternoon job into a machine-shop nightmare.
Give yourself the best odds:
- Work on a warm, not hot, engine so the aluminum has expanded slightly around the plug.
- Apply penetrating oil and let it soak, repeating over time rather than forcing it.
- Back the plug out a little, then snug it back in, working it loose gradually instead of yanking it straight out.
- Stop and reapply penetrant if it starts to bind, and never apply brute force.
When installing new plugs, use a small amount of anti-seize where the manufacturer allows it, and torque to spec, an over-tightened plug can crack and an under-tightened one can back out. If a plug does break off, stop and address it properly, because fragments in the cylinder will destroy the engine.
Frequently asked questions
Does a Cummins have glow plugs?
No. Cummins diesels use an intake grid heater to warm incoming air, not glow plugs. This guide is for Power Stroke, Duramax, and other glow-plug engines.
What resistance should a good glow plug read?
Typically a low value, often just a couple of ohms. An open circuit means the plug is dead, exact specs vary by engine.
How do I know if it is the plugs or the controller?
Test plug resistance first. If the plugs are good but get no power during the glow cycle, the controller, relay, or wiring is the culprit. Need parts? Email sales@coredieselparts.com.
How long do glow plugs last?
It varies by engine and climate, but many sets last well over 100,000 miles, while hard cold-weather use shortens that. Rather than replacing on a strict schedule, most owners test resistance when cold-start symptoms appear, white startup smoke, a rough cold idle, or a cold miss, and replace any plug that reads open or far out of line with the others. Because the plugs share age and heat cycles, if several test bad it is usually worth replacing the full set at once so you are not back under the hood a month later for the next one.
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