What does P0300 mean and where do I start?
Short answer: P0300 means the computer detected random or multiple-cylinder misfires; start by checking for cylinder-specific misfire codes and counts, then look for causes common to all cylinders.
P0300 sets when the crankshaft speed sensor sees misfires that are not tied to one cylinder, or that move between cylinders. If you also see P0301 through P0308, those point to specific cylinders. If not, use the scan tool's misfire counters in live data or mode 6 to see where the misfires are happening.
Because the problem is spread out, think about what all cylinders share: fuel pressure and volume, a large vacuum leak, a skewed mass airflow reading, low compression from timing problems, contaminated fuel, or an ignition supply issue. Check fuel trims and freeze frame first. Lean trims at idle with random misfire often mean a vacuum leak.
If misfires concentrate on one or two cylinders, swap the coil or plug with a neighbor and see whether the misfire follows the part. A flashing check engine light means the misfire is severe enough to damage the catalytic converter, so limit driving until it is fixed.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.