Getting a P0674 code on your Duramax? It’s frustrating, especially when your truck cranks forever on a cold morning or fills your driveway with white smoke. This guide walks you through exactly what’s happening, how to diagnose it correctly, and how to fix it without turning a simple glow plug swap into a cylinder head replacement nightmare. Read every section — the details here could save you thousands.
What Is the Duramax P0674 Code?
The P0674 code means your engine control module detected an electrical fault in the cylinder 4 glow plug circuit. Specifically, it found an open circuit, a voltage reading that’s off by more than 10%, or a complete loss of electrical continuity to that plug.
Your Duramax runs on compression ignition — no spark plugs, no ignition coils. Cold ambient temperatures pull heat out of the cast iron block and aluminum heads fast, which robs the compressed air of the temperature it needs to ignite diesel fuel. Glow plugs fix that problem by heating the combustion chamber directly before and during startup. When cylinder 4’s plug circuit fails, that cylinder starts cold, stays rough longer, and throws the check engine light.
One important side effect most owners don’t expect: your remote start shuts off automatically. The body control module disables it whenever an active powertrain fault code is present to protect your engine and emission systems.
Where Is Cylinder 4 on a Duramax?
GM numbers cylinders from the front of the engine (accessory drive side) toward the rear (transmission side), alternating left and right.
| Bank Designation | Engine Location | Cylinder Numbers (Front to Rear) |
|---|---|---|
| Bank 1 | Passenger Side (Right) | 1, 3, 5, 7 |
| Bank 2 | Driver Side (Left) | 2, 4, 6, 8 |
Cylinder 4 sits on the driver side, second from the front. That placement makes it more accessible than the rear cylinders near the firewall. Most technicians remove the driver side inner fender liner to get direct lateral access to the glow plug port without wrestling around charge air cooler piping and the steering shaft.
The factory firing order is 1-2-7-8-4-5-6-3. No matter which camshaft profile your engine runs, cylinder 4’s physical location and electrical designation stay the same.
Symptoms You’ll Notice With a P0674 Code
The symptoms depend heavily on how cold it is outside. In mild weather, you might barely notice anything. Below freezing, the symptoms get obvious fast.
- Extended cranking on cold starts — cylinder 4 has no pre-heat, so the engine struggles to fire it up
- Rough idle immediately after startup — a localized cold misfire in cylinder 4 creates an unbalanced idle that usually clears once the block warms up
- White or gray exhaust smoke — unburned diesel passing through the exhaust from incomplete combustion in cylinder 4
- Check engine light on — the PCM stores the P0674 fault and illuminates the MIL
- Glow plug indicator flashing or staying on — a solid or flashing coil symbol on your dash signals a pre-heat circuit fault
- Remote start disabled — this catches a lot of Duramax owners off guard
In late-model trucks, running with active emission codes for too long can also disrupt diesel particulate filter regeneration cycles, potentially triggering reduced-power mode. Don’t ignore this one.
How to Diagnose the Duramax P0674 Code Step by Step
Replacing parts without testing first is how you waste money. The fault could be the glow plug itself, the wiring harness, the connectors, a blown fuse, or the glow plug control module. Work through each one systematically.
Step 1: Disconnect Both Batteries First
The glow plug system pulls serious amperage. Before you touch anything, disconnect the negative cables from both batteries. Accidentally grounding a live supply wire during testing can instantly destroy the solid-state drivers inside the glow plug control module or the PCM. That’s an expensive mistake.
Step 2: Test Glow Plug Resistance
This is your first real test. Pull the power wire off the cylinder 4 glow plug and set your digital multimeter to the lowest resistance scale.
- Touch one lead to a clean, unpainted ground on the engine block
- Touch the other lead directly to the electrical terminal on top of the glow plug
A healthy Duramax glow plug reads between 0.5 and 1.8 ohms at room temperature. If your meter shows infinite resistance (open line), the internal heating element is broken. If it reads higher than 1.8 ohms, the plug is heavily degraded and needs replacement.
Step 3: Check the Wiring Harness
If the plug passes the resistance test, the problem lives in the circuit delivering power to it. Perform a continuity check on the harness between the glow plug control module and the cylinder 4 terminal.
Terminal fretting is a well-documented issue on Duramax harnesses. High-frequency engine vibration causes microscopic wear on connector pins, adding enough resistance to the circuit to push it past the PCM’s 10% tolerance threshold. A broken wire hidden inside intact insulation can do the same thing. If you find damaged sections, cut them out and splice in a new OEM pigtail connector. Always finish with dielectric grease to block moisture and slow future pin wear.
Step 4: Test Glow Plug Control Module Output
If the plug and harness both check out, the control module is the likely culprit. Its internal solid-state transistor drivers can fail from heat, power surges, or cracked solder joints on the circuit board.
Clamp an inductive digital clamp meter around the cylinder 4 power wire. With the batteries reconnected, command the glow system on during a cold crank event. A healthy circuit pulls a large initial surge — often 10 to 15 amps — that tapers to a 2–6 amp steady state as the plug heats up and its resistance rises. If you see zero current output from a module with intact supply fuses, the internal driver has failed.
Generation-Specific Differences That Change Your Repair
The P0674 fix isn’t identical across all Duramax generations. Here’s what changes between them.
| Engine RPO Code | Model Years | ACDelco Glow Plug Part | GM Part Number |
|---|---|---|---|
| LB7 | 2001–2004 | 62G | 97226202 |
| LLY | 2004.5–2005 | 63G | 97326305 |
| LBZ, LMM, LML | 2006–2016 | 9G | 12639701 |
| L5P | 2017–Present | 68G | 12725202 |
LB7 and LLY (2001–2005): Check the 175-Amp Mega Fuse First
These early engines use a straightforward control strategy. If you replace the module on an LB7 or LLY, no programming is required — the new unit works out of the box.
Before doing anything else on these platforms, check the 175-amp mega fuse in the driver side under-hood fuse block. This single fuse protects both the glow plug system and the intake air heater. If it’s blown, no cylinder gets pre-heat power, and you’ll have multiple codes instead of just P0674.
LBZ, LMM, and LML (2006–2016): Module Programming Required
Starting with the LBZ, GM integrated injector flow rate data into the glow plug control module’s memory. Replacing the module without programming it leaves you with continued fault codes and poor performance.
After installing a new module, you must use a factory-level scan tool to copy the injector flow rates from the ECM into the new glow plug control module. Skip this step, and the module won’t function correctly.
L5P (2017–Present): Three Unique Failure Points
The L5P platform adds complexity at every layer. There are three things specific to this generation you need to know.
1. Battery Distribution Unit failures
The L5P routes glow plug power through a Battery Distribution Unit. Internal fuse failures here trigger multiple simultaneous cylinder codes in specific patterns:
| Fault Location | Associated Codes | Affected Cylinders |
|---|---|---|
| BDU F5 Fuse | P066A, P067A, P067E, P068C | 1, 4, 6, 7 |
| BDU F6 Fuse | P066C, P066E, P067C, P068E | 2, 3, 5, 8 |
| X950D 55-Way Connector | P0671–P0678 | Isolated to damaged pin |
If your P0674 shows up alongside codes for cylinders 1, 6, and 7, inspect the BDU before touching a glow plug. A fault here means replacing the entire BDU assembly. Also check the X950D 55-way connector for bent or fretted pins — GM’s own technical service bulletins identify this connector as a primary cause of isolated single-cylinder open circuit codes.
2. Software-triggered false codes
GM has issued multiple TSBs covering erroneous glow plug codes on L5P engines where the plug, harness, and module are all physically fine. A software anomaly inside the control module triggers a false circuit fault. The fix is reflashing the module with the latest calibration via Techline Connect or SPS. Keep a stable power supply connected throughout the flash — a voltage drop mid-process permanently corrupts the module.
3. The Glow Plug Learn Procedure
Replace a glow plug on an L5P, and you’re not done. The L5P actively maps the baseline resistance of each plug. Install a new cylinder 4 plug without running the Glow Plug Learn procedure through a bi-directional scan tool, and the module flags the new plug’s different resistance as a fault. You’ll get the check engine light back on immediately after your repair.
How to Replace the Cylinder 4 Glow Plug Safely
This is where most DIY repairs go sideways. Follow these steps carefully.
Penetrant Soak — Don’t Skip It
Glow plugs live in an environment of extreme heat cycles and carbon accumulation. A swollen tip can seize so tightly that forcing it out with a wrench shears the plug body, leaving the threaded portion stuck in the head. Apply penetrating fluid to the threads and let it soak. Working on a slightly warm (not hot) engine block helps — aluminum expands slightly, reducing clamping force on the steel threads.
Ream the Bore Before Installing Anything
This step is non-negotiable. Carbon builds up in the glow plug bore and creates a tight interference fit around the new plug’s heating element. Force a new plug into a carbon-packed bore and you apply severe lateral pressure that can snap the tip instantly. A fractured ceramic or metal tip falling into the combustion chamber destroys your engine on the next revolution.
Use a Duramax-specific bore reamer like the Merchant Automotive 25003 or OTC 7223, machined to the precise M10x1.25 bore dimensions. Here’s the reaming protocol:
- Pack the reamer’s cutting flutes with heavy grease — this traps carbon shavings instead of letting them fall into the cylinder
- Thread the tool in fully by hand only — never use a power tool
- Turn the reamer to shave carbon off the bore walls
- Pull it out, wipe it clean, re-grease, and repeat until it spins freely
- Vacuum out any remaining debris thoroughly
Torque Specifications — Get These Right
| Component | Imperial Torque | Metric Torque |
|---|---|---|
| Glow plug body | 13 ft-lbs (156 in-lbs) | 18 Nm |
| Electrical terminal nut | 18 in-lbs | 2 Nm |
| Control module mounting bolt | 89 in-lbs | 10 Nm |
Apply a thin coat of high-temperature anti-seize to the threads before installation — it prevents galvanic corrosion between the steel plug and aluminum head. Over-torquing the plug body crushes the internal insulation. Over-torquing the terminal nut snaps the stud off the top of the plug. Use a proper torque wrench for both.
If the Plug Breaks Off: Extraction and Thread Repair
A sheared glow plug isn’t the end of the world — but it does require specialized tools and patience.
Extracting a Broken Plug
Start by removing the hardened central electrode using a twist-off extraction socket designed to grip and snap it cleanly. Standard drill bits deflect off it and chew up your aluminum head. Once the electrode is out, use a titanium-nitride coated left-hand drill bit to bore out the remaining steel shell. A left-hand tap threads into the hollowed shell, and a slide hammer pulls the seized tip free.
Restoring Stripped Threads With a Time-Sert 5000
If the threads in the cylinder head are damaged beyond repair, the Time-Sert 5000 kit is the correct fix for the Duramax platform. Unlike wire-coil inserts that can unwind under thermal cycling and compression pressure, Time-Sert inserts are solid steel bushings that mechanically lock into the aluminum permanently.
The kit accounts for the generational differences in Duramax head castings — early LB7 heads have threads starting about 6mm below the hex surface, while later heads recess them 25mm deep. The stepped drill bits and adjustable stop collars in the kit handle both.
Critical cleanliness steps during thread repair:
- Rotate the crankshaft manually until both valves for cylinder 4 are fully closed before drilling
- Push a grease-coated tissue or foam plug down the bore to block the combustion chamber before cutting
- Vacuum all aluminum shavings out completely before removing the tissue barrier
- Apply high-temperature retaining compound to the insert before driving it in
The installation driver cold-rolls the lower threads of the insert outward into the aluminum, creating a permanent mechanical lock that withstands full combustion pressures.
Fixing the duramax P0674 code right means testing before replacing, understanding what generation you’re working on, and respecting the mechanical fragility of the glow plug bore. Do it methodically, and cylinder 4 will fire cleanly again — even on the coldest mornings.
