P0300 (Random/Multiple Misfire) – Vauxhall strategy
Use counters, load, trims and simple swaps to make the fault show its hand.
Pattern recognition
- Idle-only misfire: often air leak/PCV, injector balance, or weak coil/plug.
- Load misfire: plug gap/coil breakdown, boost leak, or fuel pressure/volume.
- Cold-start rough: trims adaptation, moisture in ignition, or an intermittent coil.
Data that matters
Read misfire counters by cylinder, then compare trims and fuel pressure (where available). A random misfire with high positive trims is not ‘random’ — it’s a mixture problem.
Don’t skip the simple proof
- Swap coil/plug to see if the miss follows.
- Smoke test intake and PCV system.
- Verify MAF plausibility.
Related code
If trims are high, treat it like a lean fault until proved otherwise.
Open P0171 (Vauxhall) →Make it Vauxhall-specific
Start at the manufacturer hub to match the code to the engine family (EcoTEC petrol vs CDTi diesel) and the common platform patterns.
Open Vauxhall/Opel diagnostics hub →