Random / Multiple Cylinder Misfire Detected

The PCM watches crankshaft acceleration between every power stroke. When a cylinder fires correctly, the crank speeds up a known amount. When it misfires, the crank doesn't accelerate like it should — the PCM catches that hiccup and counts it. P0300 means misfires are happening across multiple cylinders (not just one), which usually points to something common to all of them: fuel pressure, a big vacuum leak, bad gas, low compression from a timing issue, or ignition power problems. If you ALSO have a P030X (P0301–P0308) along with P0300, focus on that specific cylinder first — that's the worst offender.

Crankshaft deceleration events tracked per cylinder. When misfire counts exceed the emissions threshold across more than one cylinder in a 200-rev or 1000-rev window, P0300 sets. Catalyst-damage threshold flashes the MIL.

Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.

1. 1Pull misfire counters by cylinder. If one cylinder is way ahead of the others, treat it as P030X (see specific cylinder code).
2. 2Check fuel trims. Big positive LTFT on both banks at idle = vacuum leak or fuel delivery. Big negative = stuck-open injector or high fuel pressure.
3. 3Verify fuel pressure with a gauge under load, not just key-on. A pump can hold static pressure but fall off when the engine asks for fuel.
4. 4If the vehicle was recently fueled — drain a sample and look/smell for water or diesel-in-gas contamination.
5. 5Scope the crank and cam signals together. A jumped tooth on the timing belt/chain will cause random misfires across the board.
6. 6Smoke-test for vacuum leaks. P0300 + P0171/P0174 together = vacuum leak almost every time.
7. 7Last resort: compression test. If multiple cylinders are low, you're looking at a mechanical issue (timing, valves, head gasket).

• Low or unstable fuel pressure (weak pump, plugged filter)
• Big vacuum leak — intake gasket, brake booster, PCV
• Bad fuel (water, contamination, wrong octane)
• Worn-out plugs across the board / wrong gap
• Failed ignition coil pack (waste-spark) or distributor cap/rotor
• EGR valve stuck open dumping exhaust into all cylinders
• Timing belt/chain off a tooth — affects all cylinders
• Low compression across multiple cylinders

Plugs $80–$300. Coil pack $150–$500. Fuel pump $400–$900. Vacuum/intake leak $100–$700. Timing job $800–$2,500. Mechanical engine work $2,500+.