Where is cylinder 3 on a 3-cylinder engine?

It is the last cylinder in the row, at the gearbox end of the block, with cylinder one at the opposite end where the timing belt or chain and the auxiliary drive sit. On a transverse installation that puts cylinder three at the side of the engine bay nearest the gearbox, which on most of these small cars is also the more awkward side to reach.

Can a wet timing belt cause a misfire?

Yes. Several small turbocharged three-cylinder petrol engines drive their camshafts with a belt that runs inside the engine, lubricated by oil rather than by running dry. As the belt ages it sheds material, which can block the oil pump pickup and starve the engine, and the belt itself can stretch enough to shift the cam timing. That shows up as misfires, often with a camshaft-to-crankshaft correlation code such as P0016 stored alongside. If your car uses one, check whether the belt has been changed at the manufacturer's revised interval rather than the original longer one.

I have P0302 and P0303 together — what does that mean?

On an inline four, two adjacent middle cylinders misfiring together is the classic signature of a failing head gasket. The strip of metal between the two centre bores is the narrowest and hottest part of the block, so that is where the gasket goes first, and when it does both cylinders lose compression at once. Before spending anything on coils or plugs, have the coolant tested for combustion gases and look for coolant disappearing with no visible leak.

Will a P0303 fail an MOT?

Yes. The illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, so the car fails on the lamp alone. A dead cylinder also raises the hydrocarbon reading at the emissions test, and on a three-cylinder engine, where one cylinder is a third of the total, that reading is usually far over the limit.

Powertrain code · standard

P0303Cylinder 3 is misfiring

Standard definition: Cylinder 3 Misfire Detected

Last updated 27 July 2026

Quick answer

Cylinder three is missing combustion events. On the three-cylinder engines now fitted to a great many small cars this is the last cylinder in the block and a full third of the engine, so the same fault feels far worse than it would on a four.

How the car works this out

The misfire is measured as a shortfall in crankshaft acceleration after cylinder three should have fired, attributed using the camshaft signal. A three-cylinder engine makes that measurement harder than a four: it fires every 240 degrees of crankshaft rotation rather than every 180, so the crank speed naturally swings much more between events even when everything is healthy. The control unit copes by comparing each cylinder against what it normally contributes, but it does mean a three-cylinder with a worn dual-mass flywheel, or with its crankshaft sensor adaptation not relearned after work, is more prone than most to reporting a misfire that is not really there.

What you might notice

A pronounced shake at idle, much more obvious on a three-cylinder than the same fault on a four
A hollow, uneven exhaust note at idle
A clear loss of power, since a third or a quarter of the engine is contributing nothing
The engine management lamp on, flashing if the cylinder is completely dead

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 spark plug for that cylinderCommon
How to check: Swap the coil with a neighbour and see whether the code follows it. On a three-cylinder there are only two neighbours to choose from, which makes the test quicker rather than harder.
2A blocked or leaking injector, particularly on the small direct injection turbo engines where injector deposits are commonPossible
3On the small turbocharged three-cylinder petrol engines that drive the camshafts with a belt running in engine oil, a degrading belt shedding material into the sump and allowing the cam timing to driftPossible
How to check: Look for a correlation code such as P0016 stored with the misfire, check the service history for whether the wet belt has been replaced at the revised interval rather than the original one, and have the oil pickup strainer inspected for shed rubber. This is a known weakness on several of these engines and it is worth ruling in or out before spending on ignition parts.
4A failing head gasket between cylinders two and three on an inline four, which is where they usually goLess common
How to check: Only really in play if P0302 is stored alongside. A combustion gas test on the coolant will confirm or clear it.
5Low compression from a burnt or tight exhaust valve on that cylinderLess common

What to do about it

1Work out how many cylinders your engine actually has before you start counting along the block. On a three-cylinder, cylinder three is the one nearest the gearbox and the last in the row.
2Swap the coil with a neighbouring cylinder and see whether the misfire moves with it. Then the plug, then the injector.
3On a small turbocharged three-cylinder, check the belt history before spending on anything else. A wet timing belt that has been left beyond its revised interval is a known problem on several of these engines and it takes the oil pump and the cam timing with it.
4If P0302 is stored alongside, have the coolant tested for combustion gases before buying ignition parts.

Can you keep driving?

Only to a garage

A three-cylinder engine running on two is genuinely unpleasant and slow, and pulling out into traffic with it is the practical risk rather than any mechanical one. Mechanically, the concern is the same as any misfire: unburnt fuel destroying the catalytic converter. If the code appears alongside a cam or crank correlation code on a wet-belt engine, do not drive it at all until the belt has been inspected.

Will it fail an MOT?

Small three-cylinder cars fail this test more often than their mileage suggests they should, because so many of them do short urban journeys and their spark plugs go out of date long before they go out of mileage. The lamp is a major defect on petrol cars first used from 1 July 2003, so the car fails on the lamp before the tester even reaches the exhaust probe. The emissions side is harsher on a three-cylinder than on a bigger engine: one dead cylinder out of three is a third of the fuel leaving the engine unburnt, which puts the hydrocarbon reading a long way over the default fast idle limit.

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

Where is cylinder 3 on a 3-cylinder engine?: It is the last cylinder in the row, at the gearbox end of the block, with cylinder one at the opposite end where the timing belt or chain and the auxiliary drive sit. On a transverse installation that puts cylinder three at the side of the engine bay nearest the gearbox, which on most of these small cars is also the more awkward side to reach.
Can a wet timing belt cause a misfire?: Yes. Several small turbocharged three-cylinder petrol engines drive their camshafts with a belt that runs inside the engine, lubricated by oil rather than by running dry. As the belt ages it sheds material, which can block the oil pump pickup and starve the engine, and the belt itself can stretch enough to shift the cam timing. That shows up as misfires, often with a camshaft-to-crankshaft correlation code such as P0016 stored alongside. If your car uses one, check whether the belt has been changed at the manufacturer's revised interval rather than the original longer one.
I have P0302 and P0303 together — what does that mean?: On an inline four, two adjacent middle cylinders misfiring together is the classic signature of a failing head gasket. The strip of metal between the two centre bores is the narrowest and hottest part of the block, so that is where the gasket goes first, and when it does both cylinders lose compression at once. Before spending anything on coils or plugs, have the coolant tested for combustion gases and look for coolant disappearing with no visible leak.
Will a P0303 fail an MOT?: Yes. The illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, so the car fails on the lamp alone. A dead cylinder also raises the hydrocarbon reading at the emissions test, and on a three-cylinder engine, where one cylinder is a third of the total, that reading is usually far over the limit.

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