The LML Duramax (2011-2016) uses glow plugs to aid cold starts, managed by a glow plug control module (GPCM). When the glow plug indicator stays on longer than normal, flashes, or a related warning appears, the module has usually detected a fault in the glow plug circuit. Here is how to read a glow-plug light that will not behave on the LML.
Common causes on the LML Duramax
- One or more failed glow plugs - the most common cause. An open or shorted plug makes the GPCM flag the circuit and light the indicator.
- Glow plug control module (GPCM) fault - the module that powers and monitors the plugs can fail and set glow-plug codes.
- Wiring, connector, or harness issues at the glow plugs, which live under the valve covers on the Duramax and can suffer heat and corrosion damage.
- Blown fuse or relay in the glow plug supply.
- Coolant/intake temperature sensor fault that confuses the glow strategy.
How to diagnose it
- Scan for glow plug and GPCM codes (P0670 range and cylinder-specific glow plug codes) - they usually name the failed circuit or plug.
- Test each glow plug's resistance; an open or out-of-spec plug is the usual find. Under-valve-cover access makes this the main labor.
- Check GPCM operation and its power and ground, and confirm it is commanding the plugs.
- Inspect wiring, connectors, fuses, and the relay in the glow circuit.
- Verify the coolant/intake temperature sensor data is sane so the strategy runs correctly.
Fixing it
Replace failed glow plugs - many owners do the full set while the valve covers are off, since the labor to reach them dominates the job. Replace a confirmed faulty GPCM, and repair any damaged wiring, connectors, fuses, or relay. Fix a bad temperature sensor so the glow strategy times correctly. Because these are genuine glow plugs, the LML diagnosis is a proper glow-plug path - it does not translate to a Cummins grid-heater setup. Once the circuit is healthy, the indicator returns to its normal brief cycle and cold starts firm up.
Frequently asked questions
Does the LML Duramax use glow plugs or a grid heater?
The LML Duramax uses glow plugs, controlled by a glow plug control module. A misbehaving glow-plug light points at the plugs, the module, or the wiring - not a grid heater.
Why does the glow plug light flash or stay on?
The control module lights or flashes the indicator when it detects a fault in the glow plug circuit, most often a failed plug. A scan tool will usually name the affected cylinder.
Should I replace all the glow plugs at once?
It is common to. The glow plugs sit under the valve covers, so most of the cost is labor to reach them - replacing the full set while you are in there avoids paying that again soon.
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