Diesel Trouble Codes

Hard Start or Long Crank — L5P Duramax

Diagnostic reference for hard start/long crank on 2017–present 6.6L L5P Duramax. Focuses on fuel supply/rail pressure and cold-start glow system faults that…

Updated October 15, 2026

What’s going on

On the L5P, a hard start or long crank usually means the engine is not reaching start-enabling fuel rail pressure quickly enough, or cold-start combustion is weak due to glow system faults. Typical contributors are low fuel supply/air in fuel, fuel pressure regulator/control issues, and weak glow plugs or glow plug module problems in the cold.1

Most likely causes

  1. Low fuel supply or air ingress causing slow rail pressure build — Restricted or failing supply components (lift pump, filters) and air ingress reduce volume to the Denso HP4 pump, delaying rail pressure rise on crank. Often sets P0087 low rail pressure.12
  2. Fuel pressure control valve / regulator circuit fault — The pump metering valve controls rail pressure. Electrical or mechanical faults in the control circuit or valve can prevent rapid pressure build. Related codes P0089, P0090.12
  3. Weak glow plugs or glow plug control module fault — Cold-start misfire/roughness from insufficient pre-heat. Glow system faults commonly set P0380 and P0670 on the L5P.1
  4. Excessive leak-back or injector sealing issue affecting start pressure — Leaking injectors can bleed rail pressure during crank, lengthening start time. May appear with rail pressure low codes.1

How to narrow it down

  1. Read stored and pending trouble codes; note P0087, P0089, P0090, P0380, P0670.
  2. Check fuel supply health: lift pump output volume/pressure, filter condition, and signs of air ingress in supply lines and filter housing.
  3. Monitor rail pressure during cranking with a scan tool; compare to start-enable threshold and look for slow rise or pressure drop after key-off.
  4. Inspect glow plug system: control module power/ground, activation time, and individual glow plug resistance/circuit integrity; verify cold-start enable conditions.
  5. Perform injector leak-back evaluation and cylinder contribution/balance test to identify excessive bleed-off affecting rail pressure on start.

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
Fuel filter service$30–$1500.3–1 hrsEasy DIY
Glow plugs (full set)$80–$2502–6 hrsDIY with tools and a manual
Fuel pressure control valve (FCA/regulator)$100–$4000.5–2 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 rail pressure does not reach spec during cranking, or high-pressure fuel system service is required, have a qualified diesel technician perform diagnosis and repairs. High-pressure fuel work requires specialized tools and safety precautions.

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 symptom seed — L5P Duramax (curated) ↩︎ ↩︎