🧊 Cold starts got you cranking forever? Misfires when it's chilly? White smoke like you’re in a fog machine competition?

You might be dealing with failing glow plugs — a super common issue on Duramax diesels, especially during the colder months. While they're often overlooked compared to injectors or turbos, glow plugs play a critical role in cold starting, combustion efficiency, and emissions.

Here are the top signs your Duramax glow plugs are on their way out — and what to do about it before you end up stranded in the driveway.

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❄️ 1. Hard Cold Starts (Long Crank Time)

The most common symptom? Extended cranking before the engine fires — especially when temps drop below 40°F. On a healthy Duramax, the engine should start almost instantly after the glow plug cycle.

If you're holding the key for more than 3–5 seconds, your plugs aren’t doing their job.

Why it happens: Glow plugs preheat the combustion chamber for ignition. No heat = no fire = long crank.

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🚬 2. White Smoke on Startup

Seeing white smoke pouring from the exhaust when cold starting? That’s raw diesel fuel being injected into a cold cylinder without ignition. Unburned fuel leaves as vapor or smoke until the cylinder finally warms up.

Note: This is not the same as condensation steam — real white smoke smells like diesel and lingers.

This is one of the earliest warnings of a weak or failed glow plug.

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💡 3. Glow Plug Light Stays On (or Doesn’t Come On At All)

The glow plug indicator light on your dash is supposed to illuminate for a few seconds at key-on when cold, then shut off. If it:

• Stays on longer than usual

• Flashes intermittently

• Never comes on despite cold temps

…it’s a sign the glow plug control module (GPCM) has detected a fault or isn’t getting a reading.

On LLY, LBZ, and newer Duramax engines, this could even trigger a check engine light and store codes like P0671-P0683 (cylinder-specific glow plug faults).

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💥 4. Engine Misfires at Startup (Then Smooths Out)

Another big red flag? The engine starts rough and shakes when cold — but then smooths out after a few seconds of idling.

That’s because some cylinders are firing, while others are struggling to combust due to non-functioning glow plugs. Once things heat up, the issue disappears… but the damage is still being done.

Repeated cold misfires increase cylinder wash and carbon buildup, shortening injector and piston life.

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🛢 5. Reduced Fuel Economy & Performance in Cold Weather

When glow plugs don’t preheat properly, combustion is delayed or incomplete. This leads to:

• Poor atomization of fuel

• Excess soot

• Unburned diesel being wasted

• Slow warm-up times

All of this kills your fuel economy and makes your truck feel sluggish, especially on cold mornings.

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⚠️ Don’t Ignore Glow Plug Issues!

While a single bad plug might not leave you stranded, ignoring it can lead to bigger problems:

• Flooded cylinders

• Injector fouling

• Failed emissions readiness

• Incomplete DPF regeneration (on newer models)

• Dead batteries from long cranks

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🧪 How to Test Glow Plugs on a Duramax

1. OBD-II Scanner: Look for DTCs like P0671–P0683

2. Ohm Test (Resistance Check): Unplug the glow plug harness and test each one.

   • Normal range: ~0.5 to 2 ohms

   • Open circuit = failed glow plug

3. 12V Test Light: Ground the clip and probe each plug — if the light doesn’t illuminate, it’s bad.

4. Check the Glow Plug Control Module (GPCM) – Sometimes it’s not the plug, it’s the controller.

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🔧 Duramax Glow Plug Quick Reference

Pro Tip: Always replace glow plugs as a full set if they’re original. If one failed, the rest aren’t far behind.

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🧰 Fixing It the Right Way

• Replace ALL glow plugs (don’t just fix one)

• Check and potentially replace your glow plug controller

• Inspect the harness for corrosion or broken pins

• Use OEM-grade parts — cheap plugs can swell or break inside the head

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🛠 Need Parts?

At PistonTwistin.org, we stock OEM-quality glow plugs, controllers, and diagnostic tools to help you fix it once and fix it right. Whether you’re driving an LB7 or an LML, we’ve got what you need.

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🧠 Final Word

Glow plugs might not sound like a big deal, but in cold weather, they’re the heartbeat of your diesel’s start-up. Ignore them, and you risk long cranks, rough idle, poor mileage, and expensive repairs down the road.

Don’t wait for winter to find out yours are bad.

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