P0300 on Toyota – Random/Multiple Misfire (Platform Behaviour)

Toyota-focused notes for P0300: how misfires present, what data to use, and how to separate ignition, air and fuel efficiently.

How Toyota P0300 tends to show up

  • Often mild: the ECU sees misfires before the driver can feel them.
  • Common on cold starts, then improves as the engine warms.
  • Frequently paired with fuel-trim faults (P0171) when unmetered air is involved.

Fast split: cold-start vs load

  • Cold-start roughness: ignition quality, injector spray pattern, and small intake leaks are prime suspects.
  • Misfire under load: coils breaking down, plug gaps, and lean pockets (air leak/MAF drift) move up the list.

Common causes (in the order worth checking)

  • Incorrect/aged spark plugs (wrong heat range or gap can create borderline misfire).
  • Coil degradation under demand (often only visible on live misfire counters).
  • Unmetered air / trims elevated (PCV/intake boot) creating lean misfire pockets.
  • Fuel delivery weakness under acceleration.

Suggested test plan

  1. Check freeze-frame: coolant temperature and load when P0300 logged.
  2. Read misfire counters per cylinder (if available) and reproduce the conditions.
  3. Verify plugs/spec and inspect for oil contamination or tracking.
  4. Log trims: if P0171-style trims exist, fix airflow issues first.
  5. If a single cylinder dominates, swap coil/plug positions to see if the misfire follows.