Diesel Trouble Codes

P0300 — Random/Multiple Cylinder Misfire Detected (LLY / LBZ / LMM Duramax)

Random/Multiple Cylinder Misfire Detected

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. Fuel-rail pressure control / CP3 / regulator / injector balance issues — Common contributors on LLY/LBZ/LMM Duramax 6.6L 2004.5-2010 are fuel-rail pressure control, CP3 pump, pump-mounted fuel pressure regulator, and injector balance.1
  2. VGT/vane sticking causing airflow/EGR imbalance — VGT with electric servo and position feedback sensor can cause airflow/EGR imbalance leading to random misfire.1
  3. Cold-start glow-plug issues — Glow plugs + glow plug control module are present; cold-start misfire can occur with glow-plug circuit faults.1
  4. Turbocharger boost control position sensor/switch circuit range/performance — P003A is a valid VGT boost control position sensor/switch circuit range/performance code for this platform.2
  5. Turbocharger boost control solenoid A circuit/open — P0045 is a valid turbocharger boost control solenoid A circuit/open code for this platform.3
  6. Injector circuit/open faults — Injector circuit faults are monitored per cylinder, e.g., P0201 for cylinder 1 and P0202 for cylinder 2.45
  7. Glow plug/heater circuit malfunction — P0380 glow plug/heater circuit malfunction applies to LLY/LBZ/LMM Duramax with glow plugs + control module, no grid heater.6

How to diagnose

  1. Verify code P0300 is present and check for accompanying codes such as fuel-rail pressure, VGT, injector circuit, or glow-plug codes.
  2. Review freeze-frame data for engine temperature, load, and fuel-rail pressure at misfire occurrence.
  3. Inspect fuel-rail pressure actual vs desired using scan tool; note CP3 pump and pump-mounted regulator behavior.
  4. Check VGT operation: monitor boost control position sensor feedback and solenoid command; look for P003A/P0045.
  5. Test glow-plug system and control module for circuit faults, especially on cold start.
  6. Perform injector electrical tests and contribution/balance tests per cylinder; check for P0201–P0208.
  7. Inspect air metering (MAF/MAP) and EGR system for imbalance affecting multiple cylinders.

Parts and tools you may need

Rough cost and difficulty

Costs vary by region and labor; obtain local quotes.

When to see a pro

If misfire persists after basic checks, have a qualified diesel technician perform advanced diagnostics on high-pressure fuel system and VGT components. Do not attempt step-by-step repair of safety-critical systems (brakes, high-pressure fuel lines).

Sources

  1. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

  3. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

  4. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

  5. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

  6. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎