Symptoms / what you may see
Flashing MIL, rough idle, vibration under load, fuel smell, catalyst smell, poor power or hard start.
P0300 — Random/Multiple Misfire
Advanced code-specific diagnostic direction. Confirm the fault with scan data, circuit testing and physical inspection before replacing components.
Safety / setup
Work on level ground, secure the vehicle, keep hands clear of belts/fans and disconnect battery power only when the procedure allows it. Record active, pending, history codes and freeze-frame before clearing anything.
Possible causes
- Ignition coil, spark plug, lead or coil boot breakdown under load.
- Injector flow imbalance, wiring fault or fuel-pressure drop.
- Vacuum leak affecting multiple cylinders.
- Mechanical compression fault, valve sealing issue or incorrect cam timing.
- EGR stuck open, contaminated fuel or software/calibration issue.
Known-fault bulletin direction
Known-pattern misfires are common on coil-on-plug systems, but cylinder 5/6 or rear-bank access problems can hide compression, injector or valve faults. Do not quote coils until the misfire follows the coil or waveform confirms it.
Confirm VIN, engine code, build date and OEM service information before quoting a known pattern failure.
Advanced test path
- Read misfire counters by cylinder and freeze-frame load/RPM.
- Swap plug/coil/injector only when one cylinder is dominant; the fault must follow the part.
- Use relative compression and leakdown if the fault stays on the cylinder.
- Scope ignition or coil current ramp for intermittent breakdown.
- Check trims, purge, EGR and fuel pressure if misfire is random across cylinders.
Repair order
- Replace proven failed ignition/fuel components.
- Repair vacuum, EGR or purge faults.
- Repair mechanical compression/timing/valve faults when proven.
- Do not run a flashing MIL longer than needed; catalyst damage risk is high.
Mobile Motive note
For Adelaide Hills, Mount Barker, Nairne, Hahndorf and surrounding bookings, use the Mobile Motive mobile diagnostics Adelaide Hills page when the fault needs scan-tool testing, electrical checks, smoke testing, pressure testing, scope capture or live-data confirmation.