The P0300 code means "random or multiple cylinder misfire detected". The engine control unit has seen that combustion is not happening properly, but it cannot pin it on a single cylinder. When it can, it logs a code in the P0301 to P0308 series, where the last digit is the cylinder: P0302 is cylinder 2, for example.
How the engine detects a misfire
Every combustion gives the crankshaft a small push. The control unit measures its speed with the crankshaft position sensor and, if at any point it slows down more than it should, it concludes that the cylinder has not burnt its mixture properly. It counts those misfires and, once they pass a limit, logs the code and turns on the warning light. If the misfires are severe enough to damage the catalytic converter, the light flashes.
Safety: with the engine warning light flashing, slow down, avoid full throttle and get the car checked. Unburnt fuel reaches the exhaust and can overheat the catalytic converter.
Most common causes
| Area | What to check |
|---|---|
| Ignition | Worn spark plugs, coils, leads or connectors |
| Fuel | Dirty or leaking injectors, low fuel pressure, blocked filter |
| Air | Intake air leaks, EGR valve not closing, dirty air flow meter |
| Mechanical | Low compression, head gasket, timing out |
| Sensors | Crankshaft or camshaft sensor giving an erratic signal |
A P0300 with misfires spread across every cylinder usually points to something they all share (fuel, air, sensors). If the misfires are concentrated on one or two, the cause is usually that cylinder's plug, coil or injector.
How to track it down with a diagnostic tool
- Read every engine fault and note whether P0301 to P0308, mixture codes (P0171/P0172) or sensor codes also appear.
- Save the freeze frame: it tells you whether the misfire happened cold, at idle, under load or at high revs.
- Check the live data with the engine warm: fuel trims, revs, load and, if your car offers them, the misfire counters for each cylinder.
- Use the misfire check or the cylinder cut-out test if your tool has them. By disabling each cylinder in turn and seeing how much the revs change, you find the one contributing least.
- Swap components to confirm: move the coil from the suspect cylinder to another one. If the misfire moves with it, you have found the culprit.
- Check that cylinder's plug and injector and, if they are fine, do a compression test.
- Repair, clear the codes and drive in conditions similar to those in the freeze frame to confirm it doesn't come back.
Switch the ignition off before touching coils or ignition leads. The ignition system runs at high voltage and can give you a shock.
Typical symptoms
- Jerking when accelerating, especially under load or uphill.
- Rough idle: the engine shakes while you wait at traffic lights.
- Loss of power and higher fuel consumption.
- A smell of fuel from the exhaust.
- Engine warning light on steady or flashing.
Pay attention to when they happen. If it only occurs from cold and goes away once warm, think injectors, plugs or an air leak that seals up as parts expand. If it happens in rain or damp weather, suspect coils or ignition leads. If it shows up at high revs or under load, check fuel pressure and ignition. That information, together with the freeze frame, saves you a lot of time.
Maintenance that prevents it
Change the spark plugs at the interval the manufacturer specifies, use good-quality fuel and check the intake hoses whenever you are working in the engine bay. On direct injection engines, carbon build-up on the valves and injectors is a common cause of intermittent misfires as the years go by.
What happens if you ignore it
Clearing the code and carrying on to see what happens is not a good idea either: if the misfire is intermittent, the light may take days to come back while the problem gets worse. A persistent misfire doesn't just sap power and raise consumption: it can damage the catalytic converter and end up triggering other codes, such as P0420. That is why, when both appear together, you always start with the misfire.
Which iCarsoft tools help with a P0300
An entry-level tool is enough to read the code and live data. For active tests by cylinder, you need a tool that includes them:
| Tool | Code reading and live data | Misfire check | Cylinder cut-out test |
|---|---|---|---|
| CR Pro | Yes | No | No |
| CR Pro+ | Yes | Yes | Yes |
| CR Max | Yes | Yes | Yes |
| CR Max P | Yes | Yes | Yes |
The CR Pro+ is the most affordable option with these tests and bidirectional control; the CR Max P adds a 10" screen and built-in DoIP (we explain what that is in what DoIP and CAN-FD are). You will find them all under multi-brand diagnostic tools. Once it is repaired, follow our guide to clearing the engine warning light.
