Diesel Trouble Codes

Rough Idle and White Smoke on Cold Start — L5P Duramax

A diagnostic reference for the L5P Duramax (2017–present) when the engine idles rough and produces white smoke on cold start, typically clearing as the engine…

Updated October 15, 2026

What’s going on

On the L5P Duramax, rough idle with white smoke on cold start usually means incomplete combustion during cold operation. The symptom commonly points to glow-plug or glow-plug module problems, an injector not firing cleanly, or the engine running too cold. White smoke and rough running that clear as the engine warms usually mean glow plug or glow plug module problems, an injector not firing cleanly, or an engine running too cold.1

Most likely causes

  1. Glow plug or glow plug control module fault — One or more glow plugs not heating or a fault in the glow-plug control module circuit reduces cold-cylinder temperature, causing misfire and white smoke until the engine warms. The PCM can set P0380 or P0670 for glow-plug circuit/module faults.12
  2. Faulty or leaking injector — A Denso common-rail solenoid injector that dribbles or fails to deliver a clean spray causes cold-start misfire and white smoke; per-cylinder contribution faults such as P0263 can appear.1
  3. Engine running too cold due to thermostat or cooling system issue — A stuck-open thermostat or cooling-system problem keeps coolant temperature low, prolonging cold-start roughness and white smoke. Low coolant temperature can set P0128.1

How to narrow it down

  1. Read stored trouble codes and freeze-frame data; note any of P0380, P0670, P0263, P0128.
  2. Verify ambient and engine coolant temperature at start; confirm symptom is cold-start specific and clears with warm-up.
  3. Inspect glow-plug control module circuit and glow plugs for faults; check for P0380/P0670 and test glow-plug operation per service information.
  4. Perform injector balance/contribution test and cylinder cut-out to identify a weak or leaking injector; correlate with P0263 if present.
  5. Check engine coolant temperature rise and thermostat operation; confirm warm-up rate and look for P0128 if the engine runs too cold.

Specs and test values

CheckNormal readingWhen to measureSource
Battery and charging voltage12.6–12.7 V at rest when fully charged; about 13.5 V runningEngine off, then running with loads onDieselHub
Glow plug resistanceLow — usually no more than about 6 Ω — and similar on every plug; an open reading means a failed plugEngine off, meter lead to engine groundDENSO

Common mistake to avoid

Replacing glow plugs or the glow module before checking the batteries. The glow system draws heavily, and low battery voltage causes circuit faults that no part swap will fix. Source: FleetRabbit — P0670 glow plug control module fault.

Parts and tools you may need

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
Thermostat$50–$1501–3 hrsDIY with tools and a manual
Glow plugs (full set)$80–$2502–6 hrsDIY with tools and a manual
Fuel injector (each)$400–$7001.5–3 hrsShop

Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If high-pressure fuel injection service is required, have a qualified diesel technician perform diagnosis and repair. Do not attempt high-pressure fuel system service without proper training and safety equipment.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources

  1. Curated symptom seed — L5P Duramax (curated) ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — L5P Duramax (curated) ↩︎