Misfires: P0300, P0301 and why you should not keep driving
The one group of codes where «it can wait until the weekend» costs more than the repair itself.
A misfire is a cylinder that took the fuel but did not burn it. The engine shakes, the light either blinks or glows steadily, and the unburnt petrol goes out into the exhaust.
The codes read simply: P0300 means misfires in several cylinders at once, P0301–P0306 name one cylinder by number. P0301 is the first cylinder, P0304 the fourth.
One cylinder: a fifteen-minute check
If the code names a single cylinder, swapping settles it. Move the ignition coil to a neighbouring cylinder, clear the code and drive a little.
- The code moved with the coil — the coil is at fault.
- The code stayed on the same cylinder — it is the spark plug, the injector or the cylinder itself.
The same is done with spark plugs. The trick is free and closes half the cases on the spot.
Several cylinders at once
P0300 with no cylinder named points at something shared: fuel, air, spark for all of them. Check in order:
- The fuel. Especially if the code appeared right after refuelling. Bad petrol is a common and annoying cause.
- An air leak. P0171 or P2187 usually sits nearby.
- All the spark plugs. If they have done more than fifty thousand kilometres, they are due anyway.
- Fuel pressure and the filter. A weak pump misfires under load.
Why it cannot wait
A separate code, P0363, says the control unit has cut fuel to the cylinder itself to save the exhaust. That is no longer a warning but a protection.
The app shows which cylinder is misfiring and whether the lamp is lit. Those two lines save a mechanic from guessing.
