How to Replace a Duramax Glow Plug Control Module

Confirm the part and repair scope

The 6.6L Duramax has been built in several generations, including the LB7, LLY, LBZ, LMM, LML and L5P, and the glow plug control module, its connectors and its mounting changed over that span. Match the replacement to your truck by VIN and engine code rather than by model year alone. The engine RPO code is listed on the service parts identification label, usually found inside the glovebox or on a door panel. A module that looks similar but comes from a different engine configuration may not plug in correctly or communicate with the engine control module.

Next, make sure the module is really the failed component. A code for a single cylinder glow plug circuit, such as one in the P0671 to P0678 range, more often points to that glow plug or its wire than to the module. Codes for the module control circuit, the module power or ground, or communication between the module and the engine computer, such as P0670 or P0683, make the module a stronger suspect, but a corroded connector, a damaged power feed or a poor ground can set the same codes. Measure glow plug resistance and harness continuity using the values in the service information for your engine before spending money on a module.

Remember that glow plugs mainly help when the engine is cold. If the truck also cranks long when warm, look at the fuel side first: low fuel pressure, air drawn in around the fuel filter housing, a weak lift or transfer fuel pump, or injectors returning too much fuel can all cause hard starts that no glow plug part will fix. A scan tool showing actual versus desired rail pressure during cranking helps separate a fuel problem from a preheat problem.

Prepare tools and safe access

Gather a scan tool that can read Duramax codes and live data, a digital multimeter, a socket set with extensions, trim and connector release tools, and a flashlight. A clamp-on amp meter is useful for verifying the glow plug circuits afterward. Get the factory or professional service procedure for your exact engine, because module location, shield removal and fastener torque differ between generations. On some trucks access is from under the vehicle, so use a level surface, wheel chocks and properly rated jack stands; never work under a truck supported only by a jack.

Duramax pickups use two batteries, so disconnect the negative cable at both of them before touching the module. The glow plug circuit carries very high current, and the module's main power feed can stay live with the key off. Let the engine and exhaust cool fully. If you are not comfortable working around heavy-gauge power cables, brittle aged connectors or tight access under the truck, this is a reasonable job to hand to a shop.

This repair should not require opening any part of the fuel system. The common-rail system operates at extremely high pressure, and lines between the high-pressure pump, rail and injectors must not be loosened as a shortcut to gain room. If the service procedure for your truck requires moving fuel lines or the fuel filter assembly, follow it exactly or have a professional do that portion. Wipe up any diesel fuel you find in the area rather than letting it reach hot exhaust parts.

Remove without avoidable damage

Photograph the module, connectors and cable routing before you start so the reinstall matches the original. Remove any splash shield or cover listed in the service procedure. Release each electrical connector by its locking tab or slide rather than pulling on the wires; connectors on older trucks are often heat-hardened and crack easily. Remove the nut or fastener on the main power terminal, keep the cable eyelet from touching any metal surface, and note where the ground connection is made, since some modules ground through their mounting while others use a separate strap.

With the electrical connections free, remove the mounting fasteners and lift the module out. Before going further, inspect the harness side closely. Green or white corrosion, pushed-back or spread terminals, melted plastic, chafed insulation or a loose power eyelet can all be the actual cause of the failure. Installing a new module onto a damaged connector or high-resistance power feed can lead to the same codes or damage the replacement, so repair any harness faults first.

Install and restore the system

Clean the mounting surface and the ground contact points down to clean metal where the ground path requires it. Position the new module in the same orientation as the old one so the connectors line up without strain, then install the fasteners and tighten them to the specification in the service information instead of guessing. Attach the power cable and ground, then seat each connector until it clicks and engage every secondary lock. Use dielectric grease only where the service procedure calls for it.

Route the harness along its original path, clear of exhaust components, steering parts and sharp edges, and refit any shields or clips you removed. Reconnect both battery negative cables. On some Duramax generations the glow plug module communicates with the engine control module, and the service information may call for a configuration or setup step with a capable scan tool after replacement. Check that requirement for your engine, complete it if listed, and then clear the stored codes.

Check the completed work

Turn the key on and watch the wait-to-start lamp behave normally. If your scan tool supports a glow plug output test, run it and confirm there are no new codes. During a glow cycle, verify voltage at the glow plugs or current draw with a clamp meter against the expected behavior in the service information, comparing all eight cylinders. The most meaningful test is a start after the truck has sat overnight in cool weather: it should start promptly with less white smoke than before.

Do a quick leak check around the fuel filter, fuel lines and any area you worked near, looking for wet spots or a diesel smell, and confirm all shields and cables are secure. Further diagnosis is needed if module or communication codes return right away, a single cylinder circuit code appears, or the engine still starts hard when warm. In that last case, return to fuel pressure, the fuel pump, the fuel filter and injector performance, since a preheat repair cannot correct a fuel delivery problem.