P0300 – Random/Multiple Misfire
On Mercedes turbo petrol engines, P0300 is frequently ignition-related (coils/plugs) or heat-soak driven before injectors become a likely culprit.
How it usually shows up
- Misfire only when hot or only on cold start
- Flashing MIL under load
- Rough idle with no obvious fuel smell
Most common causes (Mercedes)
- Weak coil packs/boots, worn plugs or incorrect plug spec
- Vacuum/unmetered air causing lean misfire under load
- Fuel delivery issues (less common than ignition)
- Sensor plausibility causing incorrect fueling
Practical test plan
- Check misfire counters per cylinder if available; note hot vs cold behaviour.
- Inspect and verify plug spec and gap; coils are common wear items.
- Check fuel trims; lean-at-idle often points to unmetered air.
- If trims and ignition are OK, move to fuel pressure checks and injector balance (where possible).
- Only then consider injectors or compression testing.
Related reading
Turbo Petrol Patterns
Common petrol drivability patterns and quick checks.
P0171
Lean running – trims-based diagnosis.
Tip: If you have multiple codes, start with the one that appeared first, then confirm with live data before replacing parts.