Diesel Trouble Codes

P0300 — Random/Multiple Cylinder Misfire Detected (LB7 Duramax)

Diagnostic reference for P0300 on LB7 Duramax (2001–2004).

Updated September 2, 2026

What it means

The engine control module has detected misfires in multiple cylinders that are not isolated to a single cylinder.1

Most likely causes

  1. LB7 injector body/return-flow failure causing fuel-in-oil and balance-rate drift — LB7 injectors are mounted under the valve covers and are a known weak point (cracked bodies, return-flow/balance-rate drift). Faults show as P0201–P0208, misfire P0300/P030x, rough running/fuel in oil/white smoke.1234
  2. Low rail pressure from CP3 pump or pump-mounted fuel pressure regulator — LB7 uses Bosch CP3 high-pressure pump, common rail, rail pressure sensor, fuel pressure regulator on the pump. Rail-pressure codes P0087/P0088, regulator/FPR codes P0089/P0090, and P1093 apply.1
  3. Fuel starvation / restriction from filter or supply — Inadequate seal at fuel filter connection can cause fuel leak and supply issues.5
  4. Crankshaft position sensor signal fault — ECM detected electrical or signal fault in crankshaft position sensor circuit.6
  5. Glow plug / GPCM circuit fault affecting cold-start combustion — LB7 uses glow plugs (8) with glow plug control module (GPCM). P0380/P0670/P067x valid. [5,7].78
  6. Fuel rail pressure sensor circuit high causing erroneous fueling — Rail pressure sensor circuit high typically presents as electrical fault in sensor/wiring rather than mechanical rail pressure issue, and can be confused with actual low/high pressure symptoms from CP3/regulator/injector return flow.9

How to diagnose

  1. Verify freeze frame and misfire counters; confirm random vs. cylinder-specific pattern.
  2. Inspect for fuel in oil, white smoke, rough idle; check injector balance rates with scan tool.
  3. Check rail pressure actual vs. desired at idle and load; inspect CP3 pump and pump-mounted fuel pressure regulator.
  4. Inspect fuel filter and connections for leaks; verify correct filter and seating of O-ring.
  5. Test crankshaft position sensor circuit and signal integrity.
  6. Check glow plug control module and glow plug circuits for faults affecting cold start. [5,7]
  7. Inspect fuel rail pressure sensor circuit for electrical faults; compare sensor voltage to spec.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by region and shop; injector replacement is labor-intensive on LB7 due to under-valve-cover location.

When to see a pro

If high-pressure fuel system diagnosis is required, seek a qualified diesel technician. Do not service high-pressure fuel lines without proper training and safety equipment. If fuel leak is suspected, stop operation and have vehicle inspected immediately.

Sources

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

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

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

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

  5. NHTSA ↩︎

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

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

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

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