P0300 - Random / Multiple Cylinder Misfire

What Is P0300?

P0300 means the ECU has detected misfires occurring across multiple cylinders or in a random, non-repeating pattern. The ECU monitors crankshaft rotation via the crankshaft position sensor - each misfire causes a tiny but measurable drop in rotational speed that the ECU can detect and attribute to a specific cylinder. When the misfire pattern does not settle on one cylinder, or affects multiple cylinders simultaneously, P0300 is stored.

P0300 is one of the most serious common fault codes. Persistent misfires send unburnt fuel into the catalytic converter, causing it to overheat and fail. On severe misfires, raw fuel entering the exhaust can also wash oil off cylinder bores and damage the engine. Identifying the root cause quickly is essential - the causes range from something as simple as a set of worn spark plugs to a failed head gasket.

Common Causes

How to Diagnose P0300

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Every P0300 outcome, written out in full

The tool above asks a couple of questions and lands you on one of these 6 results. They are all set out here as well, so you can read them without clicking through, compare them against each other, or check one on a phone with no signal.

If you answered The engine light is flashing and the car is shaking hard

Stop, do not drive A flashing engine light does not mean the same thing as a steady one. It means the misfire is bad enough right now that unburnt fuel is being pumped into the catalytic converter, and a cat can be cooked in a matter of miles.

• Dead coil pack or coil-on-plug (Most likely) A coil that has failed outright kills one cylinder completely, and that is what makes the light flash rather than glow. On a coil-on-plug engine you can often find it by swapping the suspect coil to another cylinder and seeing whether the misfire follows it. Coils are usually £25-£70 each.
• Failed or broken spark plug (Likely) Plugs left in past their interval crack their ceramic or wear the gap so wide the coil cannot jump it. If the plugs have no service history, do not chase anything else until they have been out and looked at. On most engines it is an hour and £40 in parts.
• Dead injector or its wiring (Possible) An injector that has stopped opening, or lost its plug connection, gives the same complete dead cylinder as a dead coil. Cheap check first: unplug and inspect the connector for oil, corrosion and cracked locking clips before anyone quotes for injectors.

What to do next: Stop driving it. Not tomorrow, now. A misfire like this can turn a £60 coil into a £700 catalytic converter within a single journey, and once the cat melts internally it also blocks the exhaust and can hurt the engine. Get it recovered or fixed on the drive.

Next time, read the code yourself

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If you answered Only for the first minute or two from cold

Drive carefully, book it in A misfire that only exists for the first minute from cold is telling you the engine is fine mechanically and something is wrong with the extra fuel and air the ECU adds while it warms up.

• Worn spark plugs (Most likely) A cold engine is the hardest condition for a tired plug - the mixture is rich and damp and the spark has the most work to do. Plugs that pass a warm test happily can still misfire cold. This is by far the most common answer and the cheapest to rule out.
• Coolant temperature sensor reading wrong (Likely) The ECU decides how much extra fuel to add from the coolant sensor. A sensor that reports the engine as warmer than it is starves the cold start, and a misfire follows. Live data settles it in seconds - the reading at first start should match the outside air temperature.
• Air leak that only bites when cold (Possible) Perished intake and vacuum hoses shrink and harden when cold and seal up as they warm through, so a small leak causes a misfire only in the first minute. Smoke testing a cold engine is the only reliable way to find these. How to find an air leak

What to do next: Take the plugs out and look at them before spending anything else. If they are past their interval, or two of them look different from the rest, you have almost certainly found your fault. Note which cylinder each plug came from - the odd one out points at the cylinder.

If you answered Lumpy at idle, smooths out once you are moving

Drive carefully, book it in Rough at idle and smooth once you are moving is the signature of unmetered air. At idle a small leak is a big percentage of the total airflow, so it upsets the mixture badly. At 3,000 rpm the same leak is lost in the noise.

• Vacuum or intake air leak (Most likely) Split intake elbows, perished brake servo hoses, a leaking PCV pipe and a hardened intake manifold gasket all do this. A quick tell is idle speed that hunts up and down rather than sitting steady. Find it with smoke rather than by replacing parts on a hunch. Smoke test the intake properly
• Carboned throttle body or idle air passage (Likely) Soot and oil mist build a lacquer on the throttle plate over the years and the ECU loses control of the small airflow it needs at idle. Cleaning it is a half hour job with a can of throttle body cleaner, and many cars need an idle relearn afterwards or they idle worse before they idle better.
• Carbon on the intake valves (Possible) On direct injection petrols there is no fuel washing the back of the valves, so carbon builds into a sponge that disturbs airflow at low lift. Typical on VAG TSI, BMW N43 and N47 and Ford EcoBoost past 60,000 miles. Walnut blasting is the fix and it is a garage job.

What to do next: Before you buy anything, get someone to spray a little carb cleaner around the intake joints and vacuum hoses with the engine idling. If the idle speed changes as you spray a spot, you have found the leak. Do it in daylight, keep it away from anything hot and use short bursts.

If you answered It started right after I filled up

Drive carefully, book it in A misfire that begins within a few miles of a fill-up is usually about what came out of the pump, not about the engine. That is good news, because the cure is often just getting rid of it.

• Wrong or contaminated fuel (Most likely) A small amount of the wrong grade or a bad batch upsets combustion across all cylinders at once, which is exactly what P0300 describes. If you filled at an unfamiliar station and the fault started within twenty miles, that is your prime suspect.
• Water in the fuel (Likely) Water sits at the bottom of station tanks and gets stirred up after a delivery. It causes a stumble that is worse under load and can also throw lean codes alongside P0300. Draining and refilling with fresh fuel from a busy station clears it.
• Coincidence - plugs at the end of their life (Possible) Sometimes the fill-up is a red herring and the plugs were simply due. Worth knowing before you drain a full tank of perfectly good fuel: check the service history first.

What to do next: If you are confident about the fuel, the safest move is to drain and refill rather than run it through. Modern high pressure petrol and diesel systems do not tolerate contamination well, and the cost of a drain is a fraction of the cost of an injection system. Keep your receipt - a fuel station will sometimes cover it if a batch is bad.

If you answered Under load - pulling away, hills, overtaking, then Petrol

Drive carefully, book it in A petrol that is fine gently but breaks up under load is a classic electrical breakdown pattern. Under load the cylinder pressure is higher and the spark needs far more voltage to jump the gap, so a marginal component fails only when you ask the most of it.

• Coil breaking down under load (Most likely) A coil with degraded internal insulation makes enough voltage at light throttle and gives up when the demand rises. It will often pass a static test perfectly. If the misfire follows a swapped coil to a different cylinder, that is your answer.
• Plug gap worn too wide (Likely) Every mile widens the gap a fraction. Past the service interval the coil can still bridge it at idle but not under boost or full throttle. Check the gap against the manufacturer figure rather than assuming a new looking plug is a good one.
• Fuel starvation under demand (Possible) A tired fuel pump or a filter that has never been changed delivers enough at idle and runs out of pressure when the engine wants full flow. The tell is that the misfire gets worse the longer you hold the throttle open, rather than appearing instantly.

What to do next: Get a scanner on it and watch the misfire counters while someone drives it under load. If the counts pile onto one cylinder it is coil, plug or injector on that pot. If they are spread evenly across all of them, stop looking at ignition parts and start looking at fuel supply and air leaks.

If you answered Under load - pulling away, hills, overtaking, then Diesel

Drive carefully, book it in A diesel misfiring under load is not an ignition problem, because there is no ignition system to blame. It is fuel delivery, and the amount of fuel a diesel wants under load is many times what it needs to idle.

• Failing injector (Most likely) An injector that has lost its spray pattern still runs the engine at idle and cannot deliver a clean charge under load. A back-leakage test tells you which one in about twenty minutes, and it is worth paying a garage for that test rather than replacing injectors on a guess at £150-£300 each.
• Low fuel supply pressure (Likely) A blocked filter, a tired lift pump or a collapsing suction hose all give plenty of fuel at idle and run short under load. If you also have a low rail pressure code stored, treat this as the leading theory. P0087 - fuel rail pressure too low
• Low compression on one cylinder (Possible) Worn bores, a tired valve or a leaking injector washer drop compression on one pot, and a diesel with low compression on one cylinder misfires hardest when it is working. This is the expensive answer, so prove the cheap ones are innocent first.

What to do next: Change the fuel filter first if there is any doubt about when it was last done - it is cheap, it is due anyway, and it removes the most common cause from the list. If the misfire survives a new filter, book a back-leakage test before anyone starts quoting for injectors.

P0305 and P0306 - Misfires On Cylinders 5 and 6

If you are reading P0305 or P0306, your engine has at least five cylinders - these are the same per-cylinder misfire codes as P0301 to P0304, just further along the block. Every cause and test on this page applies unchanged: coil, plug, injector, compression, in that order of likelihood. The code tells you where to look, and the swap test tells you what is wrong - move the coil from the flagged cylinder to another, clear the codes, and see whether the misfire follows.

What genuinely changes on a six cylinder engine is access. On a transverse V6 the rear bank sits against the bulkhead, and cylinders on that bank can mean removing the intake manifold or working blind by feel. Budget the time honestly before starting, identify which physical cylinder your engine calls number 5 or 6 from the workshop manual (V6 numbering schemes differ wildly between makes), and consider doing all the plugs on that bank while everything is off - nobody wants to remove the same manifold twice in a month.