P0300 Random Misfire — What It Means & When It's Serious

Common causes

• worn spark plugs (most common — check service history first)
• failing ignition coil(s)
• vacuum leak letting unmetered air into the intake
• weak fuel pump or clogged fuel filter
• dirty or failing fuel injectors
• low compression on one or more cylinders

How a mechanic diagnoses it

1. scan misfire counters per cylinder via live data
2. swap suspect coil + plug to a different cylinder and see if the misfire moves
3. fuel pressure test — key-on engine off, running, snap-throttle
4. compression test on cylinders showing misfire counts
5. smoke test the intake for vacuum leaks

Typical fix cost

$80–$600 — most common fix: spark plug + coil pack replacement (DIY-friendly for many makes). Actual cost varies by vehicle year/make and local labor rates; Fixo's AI estimate adjusts for both.

Codes commonly seen alongside P0300

• P0171 · Fuel

  P0171 System Too Lean — Common Causes & Fixes

  Why it's related: Lean fuel mixture is a common misfire cause — check fuel trims and vacuum leaks before changing plugs.

• P0420 · Emissions

  Check Engine Light P0420 — Catalytic Converter Warning

  Why it's related: Misfires that go unfixed for weeks can compound into P0420 by damaging the catalytic converter.