Fault Tree: P0300 Random/Multiple Misfire

Quick triage & tools

Capture freeze-frame: RPM, load, coolant temp, STFT/LTFT, fuel system status (open/closed loop).
Check misfire counters if your scan tool supports it (which cylinders / at what RPM).
Have ready: scan tool, multimeter, and (ideally) basic fuel pressure capability.

If you’re unsure how to interpret trims, read Fuel trim patterns (advanced) →

Decision flow

Is it a single-cylinder problem or truly random?
If you have a specific cylinder code (P0301–P0308) or one cylinder dominates misfire counts, treat it as a cylinder-specific issue (coil/plug/injector/compression).
Look for the “big 3” root causes
- Lean air leak (high positive trims, worse at idle).
- Fuel delivery (low pressure/volume; worse under load).
- Ignition weakness (worse under load / high cylinder pressure).
Fuel trims clue
- STFT/LTFT positive (lean) → check air leaks, MAF under-reporting, low fuel pressure.
- STFT/LTFT negative (rich) → leaking injector, high fuel pressure, fuel-contaminated oil, EVAP purge stuck open, O2 sensor bias.
Related: P0171/P0172 fuel trim fault tree →
Idle-specific misfire? (worse at idle / light throttle)
- Smoke test intake + PCV hoses + brake booster line.
- Check MAP at warm idle (plausibility) and MAF g/s trend.
Use: Smoke testing done right → and Vacuum testing with a gauge →
Load-specific misfire? (worse under acceleration)
- Inspect plugs (gap, heat range, oil fouling) and coil boots for tracking.
- Swap coil/plug between cylinders (if a specific cylinder starts to dominate, you found it).
A weak coil can look “random” when the ECU counts multiple cylinders, especially on turbo cars under boost.
Fuel pressure sanity
- Confirm spec at idle and under load (where possible).
- If pressure drops under load → pump/relay/filter/wiring/ground issue.
Use: Fuel pressure testing basics → and Voltage drop testing →
Injector imbalance (common “random” cause)
- Listen for consistent clicking (mechanical check).
- If you can: injector balance test / contribution test / cylinder drop.
Warning: “Fuel system cleaner” is not a test. Use it only after you’ve proven an injector issue.
Mechanical / timing sanity
- Compression test if misfire persists after fuel/air/ignition checks.
- If chain/belt timing is suspected (noise, correlation codes), check crank/cam plausibility.
Use: Crank/Cam signal checks (no scope) →
After the fix: verify
- Clear codes, re-check trims, and reproduce the exact driving condition from freeze-frame.
- Confirm misfire counters stay near zero.

Common shortcuts that waste money

Replacing coils/plugs without checking trims or leak sources.
Ignoring low voltage / poor grounds (can create phantom misfires).
Assuming the O2 sensor is the problem because trims look wrong.

Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

Freeze-frame captured / conditions noted
Battery voltage checked (resting + cranking)
Basic visual checks (hoses, connectors, grounds, fuses)
One test at a time (don’t change multiple variables)
Confirm fix by reproducing the original condition

Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.