Codes P0300 to P0306: misfires, finding the faulty cylinder

What the code means

The control unit monitors the rotation speed of the crankshaft. When one cylinder does not “push” like the others, it detects a tiny deceleration and counts a misfire. Too many misfires → code. The last digit indicates the cylinder: P0301 cylinder 1, P0302 cylinder 2, and so on. P0300 means the misfires are spread out, with no dominant cylinder.

Is it serious?

• Steady light: occasional misfires, you can drive to a garage, but do not wait for weeks.
• Flashing light: severe misfires in progress, reduce the load immediately and stop. The unburnt fuel burns inside the catalytic converter and destroys it. Read Flashing check engine light: why you must stop.

Symptoms: engine shaking at idle, jerking, loss of power, smell of petrol at the exhaust, higher fuel consumption.

Causes, by order of likelihood

For a single cylinder (P0301 to P0306)

1. Spark plug worn, fouled, incorrect gap, cracked.
2. Ignition coil (or high-tension lead on older engines): very common on engines with one coil per cylinder.
3. Clogged or failed injector.
4. Loss of compression: valve no longer closing, head gasket, piston ring. The code comes back despite a new plug and coil.
5. Localised air leak on one intake runner (on engines with individual runners).

For random misfires (P0300)

1. General air leak, MAF sensor (see P0171).
2. Low fuel pressure, clogged filter, poor-quality fuel.
3. All spark plugs at the end of their life.
4. Crankshaft or camshaft sensor dropping out, timing out of phase.
5. EGR valve staying open at idle (see P0401).

What to check first

1. Read all the codes. A P0171 or a P0087 alongside the P0300 points to the fuel supply, not the ignition.
2. The code already tells you the cylinder (P0301 = cylinder 1, P0302 = cylinder 2…): that is where to look. In the “Sensors” screen of DiagAuto AI, also look at the fuel trims: if they are strongly positive, they point to an air leak or a lack of fuel rather than the ignition.
3. Swap test (engine cold, battery disconnected if you prefer): swap the coil of the faulty cylinder with the one from a neighbouring cylinder. Clear the code, drive. If the code moves to the neighbour (e.g. P0302 becomes P0303), the coil is to blame. If not, do the same test with the spark plug.
4. If the plug and coil are ruled out, a garage will do a compression test and check the injector.

Should you clear the code?

Yes, after each swap test: it is essential to know whether the fault “follows” the part (the app clears the engine codes after confirmation, without reprogramming anything).

Indicative cost

Set of spark plugs: €20 – 80 (more for iridium plugs). Coil: €30 – 120 each. Petrol injector: €80 – 250 each. Compression test at a garage: €40 – 80. (Indicative, France 2026.)