P0300 means the engine is misfiring across more than one cylinder rather than on one in particular. You may feel shaking, hesitation or a loss of power. A flashing warning light means stop soon, because unburnt petrol reaching the catalytic converter will wreck it. Indicative UK repair cost is £40 to £600.
The word that matters in this code is random. A single-cylinder misfire points at one coil, one plug or one injector. A random misfire across several cylinders usually points at something all of them share: the air going in, the fuel arriving, or the mechanical condition of the engine. That is a different investigation, and starting it by buying four coils is how people spend £300 and still have the light on.
Can I drive with P0300?
Get it checked before driving far. With a steady light, short and gentle journeys only, and nothing long until it has been diagnosed. With a flashing light, pull over as soon as it is safe and do not keep driving. A flashing misfire light is the engine computer saying fuel is going into the exhaust unburnt, and that is how a £200 ignition job turns into an ignition job plus a catalytic converter.
What P0300 means for your MOT
There is no line in the MOT test for "misfire". There are two lines that a misfiring engine tends to trip anyway.
- The engine management light. Under section 8.2.1.2 of the DVSA MOT inspection manual, a light that is on and indicating a malfunction is a major defect. That applies to petrol cars and petrol hybrids first used on or after 1 July 2003, and to diesels first used on or after 1 July 2008.
- The emissions check. A cylinder that is not burning its fuel sends that fuel out of the tailpipe, which drives hydrocarbons up and pushes the lambda reading out of range. Both are major defects in the same section. The tester holds the engine at around 2,500rpm during that part of the test, which is often exactly where a weak coil or a tired plug starts to let go.
So a car with a live P0300 is unlikely to pass, but it is worth being precise about why: the light fails it, and the fuel it is wasting fails it. The misfire itself is not the item being measured.
What usually causes it
- Worn spark plugs or ignition coils. Plugs past their service interval give up under load before they give up at idle. On coil-on-plug engines a single failing coil normally names its own cylinder, so a random misfire more often means the whole set of plugs is equally worn out.
- A vacuum or intake leak. Unmetered air getting in after the airflow sensor leans out several cylinders at once, which is the classic reason the misfire is random rather than cylinder-specific. Perished breather hoses and hardened intake gaskets are the usual places.
- A fuel delivery problem. Low pressure, a tiring pump or a blocked filter starves every cylinder together. This version of P0300 is normally worse under load and on hills than it is sitting at idle.
- Low compression. A stretched timing chain, tight valve clearances or a head gasket leaking between cylinders. This one misfires at idle, gets no better with new parts on the ignition side, and is the reason a compression test is worth doing before a parts bill.
What it costs to fix in the UK
£40 to £600. A set of plugs on a four-cylinder petrol engine is near the bottom. Coils, injector work or an intake gasket sit in the middle. The top of the range is where compression and timing faults live, and those can run past it.
The diagnosis is usually the cheapest part of a misfire, and what a car diagnostic costs in the UK covers what garages charge to find one.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Look at the light before anything else. Steady and flashing are two different conversations, and the flashing one ends with the car parked.
- Get the misfire counts per cylinder. Most scan tools will show them. Counts spread evenly across the cylinders mean air, fuel or timing. One cylinder running away from the others means you are really chasing P0301 to P0304, and you work that cylinder instead.
- Take the plugs out and look at them. It is the cheapest honest information the car will give you. Check the gap, check the condition, and check whether one looks different from the rest.
- Read the fuel trims at idle and at 2,500rpm. Large positive trim numbers point at an air leak or a fuel supply fault, not at ignition, and that tells you to stop buying coils.
Related codes
- P0420, catalyst efficiency below threshold. If P0300 has been left alone for a while, this is often the code that turns up next, and it is the expensive one.
- P0016, crankshaft and camshaft correlation. Worth ruling out when the misfire is at idle and new ignition parts have changed nothing.
- P0442, P0455 and P0456, EVAP system leaks. Unrelated to a misfire, but they share the same warning light, so check which code you actually have before spending.
P0301, P0302, P0303 and P0304 each name a single cylinder instead, and an air leak big enough to cause a random misfire will often store P0171 alongside it.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.