Summary
P0300 means the PCM detected misfires occurring across multiple cylinders rather than a single cylinder. The most common causes are worn spark plugs or ignition coils (40%), vacuum leaks (20%), and fuel delivery issues (15%). Because the misfire is random across cylinders, the problem is usually in a shared system — ignition timing, air supply, or fuel pressure — not a single cylinder's components.
Severity: High — misfires damage catalytic converters
Safe to drive: No, not recommended — catalyst damage accumulates quickly
Repair cost: $20–$800 depending on cause
DIY difficulty: Easy to Moderate
What does P0300 mean?
Every time a cylinder fires, it produces a small acceleration of the crankshaft. The PCM tracks crankshaft speed through the crankshaft position sensor and detects misfires by identifying moments where that acceleration doesn't happen — a cylinder that should have fired, didn't.
P0300 is the "random" or "multiple" misfire code. Unlike P0301–P0308, which point to a specific cylinder, P0300 means the PCM is seeing misfires scattered across two or more cylinders. This distinction matters for diagnosis: a single-cylinder misfire usually points to that cylinder's ignition coil, spark plug, or injector. Random misfires point to something all cylinders share.
The PCM typically sets P0300 when the misfire rate exceeds 2% over a 200-revolution sample across multiple cylinders. If the misfire rate is high enough to damage the catalytic converter, the check engine light will flash — that's your signal to pull over.