P0300: Random/Multiple Cylinder Misfire Detected
The engine computer has caught more than one cylinder, or no single cylinder in particular, failing to fire properly.
What P0300 means
Your engine computer watches the crankshaft speed through the crank sensor. Every time a cylinder fires, the crank gets a small push. When a cylinder does not burn its fuel, that push is missing and the crank slows for a moment. RepairPal explains that the misfire monitor sets P0300 when this variation crosses about 2 percent, and the light starts flashing when it reaches roughly 10 percent, which is enough to damage the catalytic converter. P0300 means the misses are spread across several cylinders or jumping around, so the cause is usually something every cylinder shares: worn plugs, low fuel pressure, a vacuum leak or a bad sensor. A shop will check for cylinder specific codes, watch live misfire counters, run a fuel pressure test and smoke test the intake. If it only misfires under load, ignition parts are the usual suspect. Treat a flashing light as serious, because a ruined converter costs far more than plugs or coils.
Common symptoms
- Flashing or steady check engine light
- Rough idle or shaking at stoplights
- Jerking or stumbling when accelerating
- Lack of power and poor gas mileage
- Engine may stall at stops
Most common causes
- Worn spark plugs or bad plug wires
- Failing ignition coil(s)
- Vacuum leak from a cracked hose or intake gasket
- Low fuel pressure from a weak pump or clogged filter
- Dirty or faulty mass airflow sensor
- EGR valve stuck open
- Bad crankshaft or camshaft position sensor
- Low compression (worn rings, valves or a head gasket leak)
What to check yourself first
Read the freeze frame data with a scanner to see the RPM and load when the misfire happened. Check how old the spark plugs are and look for cracked or loose vacuum hoses. Listen for a hiss near the intake with the engine idling. If the light is flashing, do not keep driving to test it.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace spark plugs | $288-$399 | The most common fix on engines past their plug interval. Check your owner manual; many plugs are rated for 60,000 to 100,000 miles. |
| Replace ignition coil | $238-$320 | Usually priced per coil. Shops often find one or two bad coils on a multi-misfire engine. |
| Replace intake manifold gasket | $683-$834 | Needed when a smoke test finds the leak at the intake manifold gasket. |
| Replace fuel pump | $1,344-$1,513 | Only if a fuel pressure test shows the pump cannot keep up. |
| Check engine light diagnosis at a shop | $125-$183 | Worth paying for when plugs and coils are fine, so you do not throw parts at it. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0301 Cylinder 1 Misfire Detected
- P0302 Cylinder 2 Misfire Detected
- P0171 System Too Lean (Bank 1)
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
- P0335 Crankshaft Position Sensor "A" Circuit