Bad Glow Plug Symptoms in a Diesel (How I Test Them)

That rough morning start is telling you something

Here is what the data showed. A customer came in last winter with a 6.7L Powerstroke that cranked forever on cold mornings and then ran rough for the first minute. He was ready to buy injectors. I plugged in the scan tool, found two dead glow plugs, and fixed his truck for the price of a lunch order. His truck had the classic bad glow plug symptoms in a diesel, but nobody had named them for him. Glow plug problems look dramatic and fix cheap, but only if you test instead of throwing parts at the truck. Don't guess, test.

If your diesel fires up fine in August but hates January, the glow plug system is suspect number one. I am going to walk you through the symptoms I look for and the exact tests I run, in order.

Symptom 1: long cranking on cold starts

This is the big one. When glow plugs fail, the cylinders cannot reach ignition temperature, so the engine cranks and cranks before it catches. The colder it gets outside, the worse it gets. One or two dead plugs might start fine in mild weather and only show up when the temperature drops below freezing.

Pay attention to whether the glow plug coil light on your dash behaves differently. On a healthy truck it comes on, waits a few seconds, and goes out. If it flashes or stays lit after the engine is running, the system is telling you something failed.

Symptom 2: white smoke and rough running right after startup

White smoke at startup is unburned fuel, and it is a dead giveaway that one or more cylinders did not get hot enough to fire cleanly. I expect a little white puff on a freezing morning from a healthy diesel. I do not expect a cloud that lingers, followed by a shaky idle for the first minute or two. Once the cylinders warm up from combustion, the truck smooths out and runs normally. That pattern, ugly cold start then fine, screams glow plugs to me. One caveat from the bay: a clogged fuel filter can fake a rough cold start too, so if your filter has more than 15,000 miles on it, change it before you blame the plugs.

Symptom 3: codes that name the cylinder

Modern trucks watch every plug individually. When one fails, the computer usually sets a code naming it. The pattern I see most is P0671 through P0678, which correspond to cylinders 1 through 8 on a V8. On a V8 Duramax or Powerstroke, P0673 means the circuit for cylinder 3 flagged a fault. P0380 is the broader one and points at the whole glow plug or heater circuit rather than a single plug. If you see P0670 sitting next to a P067x code, that one names the control module circuit itself, so treat the module and its feed wiring as suspects, not the plugs.

This is why I always scan first. The code tells me where to aim the multimeter instead of testing eight plugs blind. Your scan tool earns its money right here.

Cummins owners, read this first

If you drive a Ram with a 6.7L Cummins from model year 2007 through 2024, you do not have glow plugs. You have an intake grid heater, a heating element grid in the intake manifold that warms the incoming air. Same job, different design. The grid heater has its own famous problem: the heater bolt can loosen over time and drop into the intake, which can wreck pistons and valves. Cummins owners do a simple jiggle test on that bolt to check if it is loose.

Cummins switched the 6.7L to conventional glow plugs starting with the 2025 model year. The new design fires up much faster in cold weather, so if you just bought a 2025 or newer Ram, the test procedures in this article do apply to you.

Step 1: scan for codes before you touch anything

Key on, engine off, plug in your scan tool and pull every code. Write down any P0671 through P0678 codes because each one names a cylinder to test first. P0380 means the fault is at the circuit level, so plan to check the relay, module, and main feed wiring.

Step 2: how to test glow plugs with a multimeter (the resistance test)

This is the test I run on every suspected plug, and you can do it with the plugs still in the head. Cold engine, ignition off. Pull the electrical connector off the top of the plug you want to test. Set your multimeter to the lowest resistance range, usually the 200 ohm setting. Touch the red lead to the top terminal of the glow plug and the black lead to a clean bare spot on the engine block or head.

A healthy plug reads low, typically 0.6 to 2.0 ohms on modern diesel applications. The exact number matters less than the pattern. Before you start, touch your meter leads together and note that reading, usually 0.1 to 0.3 ohms, then subtract it from every plug measurement. Test every plug and write down each reading. Seven plugs at 0.8 ohms and one at OL is an open circuit, and that plug is dead. All eight reading similar numbers means the plugs are fine and your problem is upstream.

Two things will lie to you on this test. A dirty terminal adds resistance and can make a good plug look bad, so clean the contact point first. And on some trucks the plugs share a bus bar that connects them together, which means you must remove the bar before testing or every reading will be meaningless.

Step 3: confirm with a current draw test

A resistance test catches dead plugs, but a plug can read fine cold and still not heat properly under load. I confirm with a clamp meter on the feed wire during preheat. Key on, watch the glow plug cycle, and look at the draw per plug. Each plug typically pulls around 10 to 15 amps when cold, so on a V8 I expect roughly 80 to 120 amps on the main feed. A dead plug pulls zero amps and stands out immediately. If every plug draws low or nothing at all, I stop suspecting plugs and start suspecting voltage supply or the controller.

Glow plug control module vs glow plugs: how I tell them apart

All eight codes at once, or no preheat voltage at the plugs even though every plug tests good, means the fault is upstream. On an older Powerstroke that is often the glow plug relay. I listen for the relay click on key on, then verify voltage at the plugs during the cycle. On newer trucks it is the glow plug control module or its wiring. Corroded connectors and chafed harnesses cause more module codes than actual module failures in my experience, so I inspect wiring before ordering a module.

One more Powerstroke quirk from my dealer days. The glow plugs only fire when the engine is cold. If the engine oil temperature sensor reads hot when it should not, the computer skips the glow cycle entirely. If your relay never clicks on a cold engine, unplugging the EOT sensor and cycling the key is a quick way to take that sensor out of the equation. It is one of the common 6.7 Powerstroke electrical headaches I chased more than once.

Replacement tips so you do not break one

Glow plugs are long, skinny, and threaded into the head. Snap one off and your cheap job becomes a very expensive job. I do this on a warm engine, not a cold one and not fully hot. Soak the threads with penetrating oil and give it time to work. Turn the plug out slowly, and if you feel it binding, stop, back it in a quarter turn, add more penetrating oil, and try again. Patience here is everything.

Thread new plugs in by hand first so you never cross thread them. Torque to your factory manual spec and reconnect each terminal firmly. A connector that is not fully seated will give you a brand new cold start problem that you built yourself.

My verdict

Glow plug diagnosis is one of the easiest wins in diesel maintenance if you follow the data instead of your instincts. Scan for codes, test resistance on every plug, confirm with a current draw check, and only then decide whether you need plugs, a relay, or a module. The whole test takes me about twenty minutes with a multimeter and a basic scan tool.

Keep your winter starts easy by reading our winter diesel survival guide, and if you own a 6.7L Cummins, brush up on that grid heater with our 6.7 Cummins reliability overview. For background on how a diesel engine actually fires without spark plugs, and the glow plug entry in our glossary, those explain the why behind the test. Don't guess, test. Your truck will thank you on the first minus 20 morning.

Frequently asked questions

What happens when glow plugs fail?

The engine cranks longer on cold starts, puffs white smoke at startup, and may idle rough or misfire for the first minute until the cylinders warm up. You will often see the glow plug coil light flashing or a check engine light with codes like P0671.

How do I know if my glow plugs are bad?

Test them with a multimeter. A healthy plug reads 0.6 to 2.0 ohms, and an OL reading means it is dead. Compare every plug against the others, because one outlier is a failed plug. A scan tool helps too: code P0671 names cylinder 1, and so on through P0678.

What should glow plug resistance be?

Most modern diesel glow plugs measure between 0.6 and 2.0 ohms at room temperature. What matters most is consistency across all cylinders. A plug that reads OL (open circuit) or much higher than its neighbors is failed and needs replacement.

Do Cummins trucks have glow plugs?

Cummins 6.7L engines from 2007 through 2024 use an intake grid heater instead of individual glow plugs. Cummins switched to conventional glow plugs starting with the 2025 model year. If you own an older Cummins and it struggles to start cold, test the grid heater and its relay, not glow plugs.

What does the P0671 code mean on a Duramax?

P0671 means the computer detected a fault in the glow plug circuit for cylinder 1. On a V8 Duramax, cylinders 1, 3, 5, and 7 sit on the passenger side. Test that plug's resistance first, then check its wiring and connector, before suspecting the glow plug control module.

Can I drive with a bad glow plug?

Yes, for a while. The truck will still run once it is warm, but every hard start punishes the starter and the battery, and unburned fuel washes oil off the cylinder walls. Fix it before the first real cold snap, because one dead plug becomes a no start at minus 20.