Which end of the engine is cylinder 4?

On an inline four it is the cylinder at the gearbox and flywheel end, with cylinder one at the opposite end where the timing drive and auxiliary belt are. On a transverse installation that puts cylinder four against the bulkhead at the back of the engine bay. On a V6 it sits on bank two under both common numbering conventions, which makes it one of the few cylinder numbers you can place on a V engine without looking it up.

Why does the same cylinder keep misfiring after a new coil?

Because the coil was not the fault, or because whatever killed the first coil is still there. The three usual answers are oil in the spark plug well from a leaking cam cover gasket, which ruins boots one after another; a poor connection at the coil plug rather than a fault in the coil itself; and a compression problem in the cylinder, which no ignition part can fix. If the misfire refuses to follow a swapped coil, plug or injector to another cylinder, stop replacing parts and have a compression test done.

Can a misfire that only happens under load still fail the MOT?

Yes, and it usually does. There is no road test in an MOT, so the tester may never provoke the misfire, but the fault is already stored and the engine management lamp is already lit. An illuminated lamp is a major defect on petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, so the car fails on that alone regardless of how it behaves on the day.

Will a P0304 fail an MOT?

Yes. The lamp is a major defect under the emissions section of the inspection manual, and that defect accounts for 371,859 failures in the DVSA data behind this site. A cylinder that is not burning its fuel also drives the hydrocarbon reading up at the fast idle emissions test, which is a separate failure in its own right.

Powertrain code · standard

P0304Cylinder 4 is misfiring

Standard definition: Cylinder 4 Misfire Detected

Last updated 27 July 2026

Quick answer

Cylinder four is missing combustion events. On an inline four that is the cylinder at the gearbox end of the block, which on most transverse installations means the one jammed against the bulkhead — the hardest to reach and, not coincidentally, the one most often left with an old plug or a badly seated coil.

How the car works this out

Same mechanism as every misfire code: a shortfall in crankshaft acceleration attributed to a cylinder using the camshaft signal. The code number itself tells you something before you open the bonnet. A three-cylinder engine cannot set it, so the engine has at least four. And if the engine is a V6, cylinder four is on bank two under both of the common numbering conventions — whether the manufacturer alternates cylinders between banks or fills one bank before starting the other — so this is one of the few positions where you can be confident which side to open.

What you might notice

A vibration at idle, sometimes more noticeable through the bulkhead and the pedals than through the seat
A stumble under load that clears when you lift off
Reduced power and worse economy
The engine management lamp on, flashing if the cylinder is dead
On a transverse four, a rattle or ticking from the back of the engine if the coil boot has been left unseated

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The ignition coil, or a coil boot that was never pushed fully home the last time the plugs were doneCommon
How to check: On a transverse engine this coil is fitted blind against the bulkhead, so it is the one most likely to have been reconnected imperfectly. Pull it, check the boot and the connector latch, and swap it with a neighbour to see whether the code follows.
2A spark plug that was skipped, or fitted without being properly torqued, because it is the awkward one at the backCommon
3Oil in the plug well from a leaking cam cover gasket, which tends to collect at whichever end the engine tilts towardsPossible
4A blocked or leaking injector on that cylinderPossible
5Harness damage at the bulkhead end of the engine, where the loom runs closest to the exhaust manifold and takes the most heat and movementPossible
How to check: Inspect the loom where it crosses the back of the head. Look for hardened, cracked insulation and rubbed-through sections against brackets.
6Low compression in that cylinder from a burnt or tight exhaust valveLess common

What to do about it

1Check whether the car has had spark plugs or coils fitted recently. Cylinder four on a transverse engine is worked on blind, and a boot left half-seated is one of the commonest reasons a car comes back misfiring days after a service.
2Swap the coil forward to a cylinder you can see easily and watch whether the code follows. Do the same with the plug.
3Inspect the wiring across the back of the engine while you are there. It is the hottest and least-inspected part of the loom.
4If nothing follows the parts, test the cylinder rather than continuing to replace things you cannot properly see.

Can you keep driving?

Only to a garage

On a steady lamp the car will get to a garage. The reason not to leave it is the catalytic converter, which takes everything cylinder four failed to burn. If the lamp flashes under acceleration and settles when you lift off, that is the misfire crossing the damaging threshold under load, and the car should be stopped rather than nursed home.

Will it fail an MOT?

There is a trap in this one worth naming. The MOT does not include a road test, so a misfire that only appears under load may never be felt by the tester at all. It makes no difference to the outcome, because the lamp is lit from the stored code and an illuminated malfunction indicator lamp is a major defect on petrol cars first used from 1 July 2003 and diesels from 1 July 2008. A car that idles beautifully on the ramp and only misfires on a hill still fails, and it fails on the lamp before the emissions probe goes anywhere near the tailpipe.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Which end of the engine is cylinder 4?: On an inline four it is the cylinder at the gearbox and flywheel end, with cylinder one at the opposite end where the timing drive and auxiliary belt are. On a transverse installation that puts cylinder four against the bulkhead at the back of the engine bay. On a V6 it sits on bank two under both common numbering conventions, which makes it one of the few cylinder numbers you can place on a V engine without looking it up.
Why does the same cylinder keep misfiring after a new coil?: Because the coil was not the fault, or because whatever killed the first coil is still there. The three usual answers are oil in the spark plug well from a leaking cam cover gasket, which ruins boots one after another; a poor connection at the coil plug rather than a fault in the coil itself; and a compression problem in the cylinder, which no ignition part can fix. If the misfire refuses to follow a swapped coil, plug or injector to another cylinder, stop replacing parts and have a compression test done.
Can a misfire that only happens under load still fail the MOT?: Yes, and it usually does. There is no road test in an MOT, so the tester may never provoke the misfire, but the fault is already stored and the engine management lamp is already lit. An illuminated lamp is a major defect on petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, so the car fails on that alone regardless of how it behaves on the day.
Will a P0304 fail an MOT?: Yes. The lamp is a major defect under the emissions section of the inspection manual, and that defect accounts for 371,859 failures in the DVSA data behind this site. A cylinder that is not burning its fuel also drives the hydrocarbon reading up at the fast idle emissions test, which is a separate failure in its own right.

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