The LBZ Duramax is renowned for its reliability, but as these trucks age, certain components, like the glow plug system, require attention. Ensuring this system is in top shape is vital for cold starts and overall engine health.

It’s easy to think about the glow plug system on your LBZ when it’s freezing outside, but the truth is, every diesel uses glow plugs to start. High compression alone isn’t enough to force ignition of the flame wall cold, so there’s the glow plug system.

Here’s the thing: if your factory glow system is weak or tired, warm weather isn’t enough to guarantee a start every time. Yes, you might hammer on it for a few minutes with the starter, but at 16.8:1, that compression ratio isn’t doing your starter any favors. Taking a couple hours one night in the garage now to replace your glow plugs and cleaning up your glow plug system is cheap insurance, so let’s dig into it in some details. It’s not hard, and having your Duramax start on the first spin every time is always a good feeling.

It’s your truck’s cold-start safety net. It’s a network of critical components that preheat the combustion chamber, making sure the engine fires up clean and fast, even when the weather says otherwise. If one piece of the system fails, cold starts get longer, rougher, and harder—and in freezing temps or tired engines, they might not happen at all.

Let’s break down the key parts:

• Glow Plugs (8 total): These pencil-sized heating elements sit in each cylinder and glow red hot (literally) to help ignite the fuel-air mixture in cold conditions. A short time after the engine starts (depending on the temperature), the Glow Plug Control Module will turn them off.
• Glow Plug Control Module (GPCM): Usually mounted on the driver-side valve cover area, this little brain box controls how long and when each glow plug is activated based on engine temperature and ambient conditions.
• Wiring Harness & Connectors: These deliver power from the control module to each plug and relay signals back to the ECM.
• Ground Connections: Grounds are often overlooked, but they’re critical. A weak or corroded ground anywhere in the system can cause uneven glow plug performance—or total failure.
• Charging System (Batteries + Alternator): If your batteries or alternator aren’t up to snuff, the GPCM won’t get the voltage it needs to do its job. This is why glow plug issues often show up alongside weak battery performance.

Understanding how these pieces interact—and maintaining each one proactively—can save you from hard starts, warning lights, and a ton of unnecessary wear on your engine.

In the rest of this article, we’ll walk you through how to inspect, test, and maintain every part of the glow plug system—not just by slapping in new plugs, but by doing the deeper work that keeps your LBZ ready to start no matter the forecast. And when it’s time to replace those plugs, we’ll show you where to get the ones we trust most.

Let’s get to it…

1. Understanding the Glow Plug System

The LBZ utilizes eight individual glow plugs, each controlled by the Glow Plug Control Module (GPCM). This module supplies power to each glow plug, ensuring efficient combustion during cold starts. Over time, components like the glow plugs, GPCM, and associated wiring can degrade, leading to starting issues.

2. Inspecting and Testing Glow Plugs

Before replacing glow plugs, it's essential to test them:

• Resistance Test: Using a multimeter, check the resistance between the glow plug terminal and a good ground. A healthy glow plug should read less than 1 ohm. Readings above this indicate a faulty plug that needs replacement.
• Visual Inspection: Look for signs of swelling or corrosion, which can impede performance.

3. Checking the Glow Plug Control Module (GPCM)

The GPCM is pivotal in managing the glow plug system. To ensure it’s functioning correctly:

• Voltage Check: Verify that the GPCM is receiving battery voltage. So, with your multimeter, check battery voltage – likely around 12.7 or so with the truck turned off, and when the key is turned on and the GPCM is energized, you ought to see nearly that exact number. More than half a volt drop can indicate wiring issues or a faulty module.
• Diagnostic Trouble Codes (DTCs): Codes like P0671–P0678 point to specific glow plug circuit issues, while P0380 or P0382 indicate broader system malfunctions.
• Programming: If replacing the GPCM, ensure it's correctly programmed to the vehicle to prevent issues like continuous glow plug operation.

4. Inspecting Wiring and Connections

Faulty wiring can mimic glow plug failures:

• Harness Inspection: Check for chafed wires, loose connections, or corrosion, especially at the glow plug terminals and GPCM connectors.
• Ground Connections: Ensure all grounds are clean and secure (see all the ground locations for your LBZ further down the page…). Poor grounding can lead to erratic glow plug operation.

5. Evaluating the Charging System

A robust charging system supports the glow plug operation:

• Battery Health: Load test both batteries to ensure they hold a charge and provide adequate voltage, and always replace dual batteries as a set. (Yes, we know that’s an expensive proposition, but adding a new battery to an old one will only ever allow you to have a system as good as the weakest one.)
• Alternator Output: Verify the alternator is charging correctly, delivering around 14.5 volts under load.
• Cable Integrity: Inspect battery cables for corrosion or damage, ensuring efficient power delivery. These trucks are pushing 20 years old now, and if you’re still running the factory battery cables, chances are, you’ve got some internal corrosion in them, driving up resistance and not allowing you to pull all the power you might need.

6. Upgrading to High-Quality Glow Plugs

Glow plugs can last for decades, but, over time, they will begin to not be as efficient. GM no longer has a specific replacement recommendation, and a lot of us figure we’re swapping them out every 50,000- 100,000 miles. The truth is, they either work or they don’t, but if you’ve just bought an LBZ and you’re handling some deferred maintenance, a set of glow plugs isn’t a bad idea, especially if you live in the colder areas. When replacements are necessary, opt for reliable components:

• Merchant Automotive 9G Glow Plugs: Designed for 2006–2015 GM 6.6L Duramax engines, these plugs offer durability and consistent performance.

You can grab a full set right Here.

7. Regular Maintenance Tips

• Routine Checks: Periodically test glow plugs and inspect the GPCM, especially before colder seasons.
• Clean Connections: Keep all electrical connections clean and free from corrosion.
• Monitor Engine Performance: Be alert to hard starts or rough idling, which can indicate glow plug system issues.
• By proactively maintaining your LBZ Duramax's glow plug system and ensuring all grounds are clean and secure, you ensure reliable starts and prolong engine life. Regular inspections, quality components, and attention to detail make all the difference.

Factory Ground Locations on the LBZ Duramax

Proper grounding is essential for the glow plug system's performance. Here are the known factory ground locations on the 2006-2007 Duramax:

1. Front Side of Frame Ears: Located at the right front cab mount.
2. Left and Right Frame Ears for Core Support Mounts: Front side, behind the bumper.
3. Rear of Left Front Frame Ear for Cab Mount: Ensures cab grounding.
4. Top of Frame Between Frame and Bed: Right above the spare tire.
5. Engine Block Grounds: Typically found on the front of the engine block, just above the upper oil pan on both the left and right sides.
6. Firewall Ground: A braided ground strap connecting the back of the driver's side head to the firewall.
7. Passenger Side Lower Engine Ground: Near the block heater element.

Regularly inspect and clean these ground points to ensure optimal electrical performance, not just in your starting system, but for all the electronics in your Duramax LBZ. We’re always amazed at how a couple hours of cleaning grounds and connections seems to magically “fix” electrical gremlins in older trucks, whether they’re oil burners or gassers.