Diesel Trouble Codes

Hard Start or Long Crank — LB7 Duramax

A concise diagnostic reference for hard start or long crank on the 2001–2004 GM 6.6L LB7 Duramax.

Updated October 15, 2026

What’s going on

On the LB7, a hard start or long crank usually means the engine is not reaching start-ready rail pressure quickly enough, or it is missing a valid crank/cam signal, or cold-start aid is weak. Typical on this platform: a long crank is usually low rail pressure from fuel supply or injector leakage, air in the fuel, a lost crank or cam signal, or weak glow plugs in the cold.1

Most likely causes

  1. Low rail pressure from fuel supply or injector leakage — CP3 pump, fuel filter, lift pump, or injector return flow prevents rail pressure from building fast enough for start.1
  2. Air in the fuel system — Air ingress at filter housing, lines, or fittings causes compressible fuel and slow pressure build.1
  3. Crankshaft position sensor circuit fault — No valid crank signal delays sync and start. Related trouble code P0335 can set. Typical on this platform: Stalling on the LB7 usually follows a loss of fuel rail pressure or a lost crank or cam position signal.23
  4. Camshaft position sensor range/performance fault — Cam signal out of range prevents proper injection timing on start. Related trouble code P0341 can set. LB7 Duramax has a single camshaft position sensor (CMP) on the front timing cover, gear-driven engine with no VVT/ phaser; P0341 is a range/performance fault for that sensor circuit.43
  5. Weak or faulty glow plugs / glow plug control module — Cold starts rely on glow plugs and the GPCM. Related trouble codes P0380 and P0670 can set.561

How to narrow it down

  1. Read stored trouble codes first; note any of P0087, P0089, P0090, P0335, P0341, P0380, P0670.
  2. Check fuel supply basics: fuel filter condition, lift pump delivery, and signs of air ingress at filter housing and lines.
  3. Monitor rail pressure with a scan tool during cranking; compare actual vs. desired pressure. Low rail pressure from fuel supply or injector leakage is typical for long crank.
  4. Inspect crankshaft position sensor circuit and signal integrity; P0335 indicates an electrical or signal fault in the crankshaft position sensor circuit.
  5. Inspect camshaft position sensor circuit and signal; P0341 is a range/performance fault for the single CMP on the front timing cover.
  6. Test glow plug system: check GPCM operation, glow plug resistance, and related codes P0380/P0670.
  7. If rail pressure is low, evaluate injector return flow/balance rates and CP3 performance; injector leakage is a known LB7 weak point.

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
Wiring or connector repair$10–$1001–4 hrsDIY with tools and a manual
Fuel filter service$30–$1500.3–1 hrsEasy DIY
Crankshaft or camshaft position sensor$40–$2000.5–2 hrsDIY with tools and a manual
Glow plugs (full set)$80–$2502–6 hrsDIY with tools and a manual

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 build during cranking, or if injector return flow testing is needed, have a diesel technician perform high-pressure fuel system diagnosis. Do not service high-pressure fuel components without proper training and safety equipment. If crank or cam sensor replacement is required, professional diagnosis of wiring and signal integrity is recommended. Glow plug and GPCM service requires proper electrical safety procedures. If the engine does not start after basic checks, seek a qualified diesel technician.

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 — LB7 Duramax (curated) ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — LB7 Duramax (curated) ↩︎

  3. Curated symptom seed — LB7 Duramax (curated) ↩︎ ↩︎

  4. Curated DTC seed — LB7 Duramax (curated) ↩︎

  5. Curated DTC seed — LB7 Duramax (curated) ↩︎

  6. Curated DTC seed — LB7 Duramax (curated) ↩︎