P0301–P0308 — Cylinder-Specific Misfire Codes

Category

Powertrain → Fuel System / Combustion

What it means (plain English)

These codes mean the ECM has detected a misfire in a specific cylinder:

  • P0301 — Cylinder 1 Misfire
  • P0302 — Cylinder 2 Misfire
  • P0303 — Cylinder 3 Misfire
  • P0304 — Cylinder 4 Misfire
  • P0305 — Cylinder 5 Misfire
  • P0306 — Cylinder 6 Misfire
  • P0307 — Cylinder 7 Misfire
  • P0308 — Cylinder 8 Misfire

On diesel engines, this usually comes down to fuel injector, compression, or fuel delivery issues rather than spark/ignition.

Symptoms

  • MIL (Check Engine Light) illuminated
  • Rough idle or engine shake
  • Loss of power under load
  • White or black smoke from exhaust
  • Hard starting if multiple cylinders misfire
  • Poor fuel economy
  • Engine may derate or go into limp mode if severe

Priority Level

High —
Cylinder misfires increase raw fuel in the exhaust, risking DPF, turbo, or catalyst damage. Long-term, misfires can cause piston or valve damage.

Common Causes

  • Faulty injector (most common on diesels)
  • Low fuel rail pressure (pump, regulator, clogged filter)
  • Injector harness wiring fault
  • Cylinder compression loss (valves, piston rings, head gasket)
  • Stuck-open EGR valve causing excess dilution
  • Contaminated or poor-quality fuel
  • Boost/air intake leaks reducing combustion efficiency

How Pros Diagnose It (step-by-step)

  1. Confirm code — see which cylinder(s) are affected.
  2. Check injector balance rates (Duramax, Cummins, Powerstroke all support this test).
  3. Cylinder contribution test — cut out each injector electronically and observe engine change.
  4. Inspect wiring/connectors to affected injector.
  5. Fuel system check — verify rail pressure and fuel quality.
  6. Compression test on affected cylinder if injector tests good.
  7. Inspect EGR valve and boost system for contributing airflow issues.

Likely Fixes

  • Replace faulty injector(s)
  • Repair/replace wiring harness to injector(s)
  • Replace clogged fuel filter or repair fuel pump/regulator
  • Repair engine mechanical issues (valves, rings, head gasket)
  • Clean or replace EGR valve if stuck
  • Fix boost/air intake leaks
  • Drain and replace bad fuel

Related/Companion Codes

  • P0300 — Random/Multiple Cylinder Misfire
  • P0200–P0208 — Injector Circuit Malfunctions
  • P2146–P2155 — Injector Group Supply Voltage Faults
  • P0087/P0191 — Fuel Rail Pressure Faults

Tech Notes (Quick Hits)

  • Duramax engines: balance rate tests are the gold standard for pinpointing faulty injectors.
  • Cummins ISX/ISB: common causes include injector connector tube leaks and rail pressure regulator issues.
  • Ford Powerstroke: known for harness/connectors causing cylinder-specific misfires.
  • If multiple adjacent cylinders misfire, suspect harness or fuel rail issues, not just injectors.

Severity: High — Repair Immediately
Codes P0301–P0308 mean one of your engine’s cylinders is misfiring. This often points to a bad injector, low fuel pressure, or compression problems. Misfires not only reduce power but also risk damage to your DPF, turbo, and engine if ignored.

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