P0301, P0302, P0303 and P0304 each name a single misfiring cylinder: cylinder 1, 2, 3 or 4 respectively. An isolated misfire is usually the coil, the spark plug or the injector on that one cylinder. A flashing warning light means stop soon. Indicative UK repair cost is £30 to £400.
These four codes are one fault with four addresses. The diagnosis is the same whichever of them you have, and the only thing that changes is which cylinder you work on. That is why they share a page rather than having four of their own: four nearly identical pages would say the same thing four times less well.
Which cylinder is cylinder 1?
This is the question that stops people, and an honest answer has two halves.
The half nobody can give you from a web page: cylinder numbering is set by the manufacturer, not by a universal rule. Most inline engines are numbered from one end to the other, and it is common for cylinder 1 to be the end furthest from the gearbox, which on a transverse engine usually puts it at the accessory belt end. The word doing the work in that sentence is common. Some engines are numbered the other way round, and on a V6 or V8 the numbers alternate between the banks in a pattern specific to that engine. Guessing from position is exactly how an expensive part ends up fitted to a healthy cylinder, so the numbering for your engine has to come from manufacturer data rather than from a rule of thumb.
The half that works on any engine, with no data at all: move a part and see whether the code moves with it. Take the coil from the cylinder the code names, swap it with the coil from a neighbouring cylinder, clear the code and drive the car. One of two things happens. Either the code follows the coil to its new position, which means the coil is the fault, or it stays where it was, which means the fault is in the cylinder and the coil is fine. You do not need to know which physical cylinder is which to run that test. You only need to be sure which two you swapped. It is the single most useful thing on this page and it costs nothing.
Can I drive with a single-cylinder misfire?
Get it checked before driving far. A single weak or dead cylinder is less alarming to drive than it sounds and more damaging than it feels. The engine runs, usually with a shake at idle that smooths out as the revs rise, and plenty of people cover weeks like that without much thought. All the while, the fuel that cylinder is failing to burn is going out into the exhaust. If the light is flashing rather than steady, the engine computer is specifically warning you about this, and the right response is to stop as soon as it is safe rather than to press on and finish the journey.
What these codes mean for your MOT
Three things worth keeping apart, and the third is the one people do not see coming.
- The lamp. Section 8.2.1.2 of the DVSA MOT inspection manual covers petrol cars first used from 1 July 2003 and section 8.2.2.2 covers diesels from 1 July 2008, and under both a lamp indicating a malfunction is a major defect. A flashing lamp is unarguably indicating a malfunction, so there is no debate to be had about that one.
- The gases. One cylinder's worth of unburnt fuel lifts hydrocarbons and pushes the lambda coefficient out of range, and each of those is a separate major defect. The reading is taken at a raised engine speed and then again once the idle has settled, and a single-cylinder misfire is usually at its worst at idle, which is precisely where that second reading happens.
- The converter, afterwards. If the misfire has already damaged the catalytic converter, you have acquired a second MOT problem that does not disappear when the misfire is fixed. Section 8.2.1.1 makes emissions equipment that is obviously defective a major defect in its own right. The MOT consequences of a misfire can outlive the misfire by a long way, and that is the strongest argument for dealing with these codes quickly.
The four codes
Here is what each one names. The diagnosis and the cost range are the same for all four. What differs is only which cylinder you are working on, and how easy that cylinder happens to be to reach on your particular engine.
P0301, cylinder 1 misfire
Cylinder 1 specifically is misfiring. Isolated misfires are usually a plug or coil on that cylinder. Indicative cost £30 to £400.
P0302, cylinder 2 misfire
Cylinder 2 is misfiring, typically a coil, plug or injector on that specific cylinder. Indicative cost £30 to £400.
P0303, cylinder 3 misfire
Cylinder 3 is misfiring, usually an ignition or fuelling fault on that cylinder. Indicative cost £30 to £400.
P0304, cylinder 4 misfire
Cylinder 4 is misfiring. Check the coil, plug and injector for that cylinder first. Indicative cost £30 to £400.
What usually causes it
- The ignition coil on that cylinder. The most likely answer on a modern coil-on-plug engine, and the one the swap test above proves or disproves for nothing.
- The spark plug on that cylinder. Cheap, and worth replacing as a full set rather than singly, because the others are the same age and have done the same work.
- The injector on that cylinder. Blocked, or leaking. On many engines injectors can be swapped the same way coils can, though it is a longer job and on direct injection engines a considerably longer one.
- A compression problem in that cylinder. A burnt valve, a worn bore, or a head gasket leaking at that cylinder. This is the version where nothing you buy on the ignition or fuel side changes anything, and it is why a compression test is the step that ends the guessing rather than extends it.
What it costs to fix in the UK
£30 to £400. A single spark plug is the bottom, and a full set is not far above it. A coil is the middle. An injector is the upper middle. A compression fault sits beyond the top of that range entirely, because that is cylinder head work and it is priced accordingly.
Most garages will charge to confirm which cylinder and which component before quoting the repair itself, which is reasonable. What a car diagnostic costs in the UK covers what that fee should look like.
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. Steady means you have time to work through this list properly. Flashing means the car gets parked until the misfire is dealt with.
- Run the swap test. As set out above. It is free, it needs no knowledge of your engine's numbering, and it either names the coil or clears it. Nothing else on this list is as efficient.
- Pull the plug from the named cylinder and compare it with its neighbours. What you are reading is the difference, not the absolute condition. Oil on that one plug points at a valve cover gasket or at something worse inside. A plug wet with fuel says ignition has stopped while fuelling has carried on. A plug that is oddly clean and dry while the other three are normally coloured points at compression, because nothing has been burning there at all.
- Compression test that cylinder if the parts have been proved good. Once the coil has been swapped and cleared, the plug changed and the injector checked, there is only one thing left and it is worth measuring rather than guessing at.
Related codes
- P0300, random or multiple cylinder misfire. The page to read instead if the misfire is spread across cylinders, because that points at something they all share rather than at one component.
- P0420, catalyst efficiency below threshold. The expensive consequence described in the MOT section above.
- P0016, crankshaft and camshaft correlation. Worth ruling out when a misfire persists after the coil, plug and injector on that cylinder have all been proved good.
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.