Ignition and misfire

P0304Cylinder 4 misfire detected

Cylinder 4 Misfire Detected

Stop driving or get it looked at now

Generic OBD-II code

Ignition/Combustion System (Misfire Monitor)

What this part does

The engine control module monitors crankshaft speed fluctuations to detect a cylinder that is not firing or burning fuel completely; the underlying cause is usually ignition, fuel delivery, or mechanical (compression/valve-train).

What the computer is measuring

The misfire monitor does not listen to the engine. It watches the crankshaft. Every cylinder that fires gives the crankshaft a small push, and the crankshaft position sensor is precise enough to resolve each of those pushes individually. A cylinder that fails to burn its charge leaves a measurable gap in that pattern, and because the computer knows the firing order it can name which cylinder was responsible.

P0304 names cylinder four in the manufacturer’s numbering, which is not always the fourth one you would count to. On an inline four it usually is, and it is normally the one at the far end from the accessory drive, tucked against the bulkhead where the engine bay is hottest and the access is worst. On a V6 or a V8, cylinder four is frequently the first cylinder of the second bank.

When it misfires is the most useful clue

A single cylinder misfire narrows the cause to the handful of things that belong to that cylinder alone. What narrows it further, and what most people skip, is the pattern.

A misfire that appears on a cold start and clears as the engine warms usually means fuel, not spark. A misfire that only appears under load, when you are climbing or overtaking, is generally ignition, because cylinder pressure makes the spark harder to establish and a marginal coil or a worn plug gap gives up first. A misfire that is at its worst at idle and smooths out as revs rise points at unmetered air, because a fixed size leak matters much less once the engine is drawing a lot more air.

Get that pattern from the freeze frame or from your own observation before you buy anything, and the swap test that follows has far less work to do.

The flashing light is not a suggestion

A steady light with this code means the misfire is intermittent and the car can be driven to be looked at. A flashing light means the misfire is severe enough that raw fuel is being pushed into the exhaust right now. That fuel burns in the catalytic converter instead of in the cylinder, and converters do not survive it for long. Stop, switch off, and do not drive it home to see if it settles down.

What you will notice

  • A flashing check engine light while it is happening, which means pull over and shut the engine off.
  • Rough idle and a vibration you can feel through the seat and the pedals.
  • Hesitation and reduced power, often worst under load or on a cold start.
  • A smell of unburnt fuel and a sharp drop in economy.

Likely causes, most common first

Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.

  1. A failed ignition coil on cylinder 4

    The most common cause on a coil on plug engine, and the easiest to prove by swapping it to another cylinder.

  2. A worn or fouled spark plug

    Cheap, common, and the deposits on the old plug tell you what caused the failure as well as confirming it.

  3. A blocked or leaking injector on cylinder 4

    Feels identical to an ignition fault from the driver's seat, so it has to be separated by test rather than by ear.

  4. A vacuum or intake port leak at cylinder 4

    Unmetered air at one runner leans that cylinder enough to misfire while the others run normally. Often worse at idle and better under load.

  5. Low compression in cylinder 4

    The expensive outcome. A burnt valve, a worn ring pack or a head gasket failure between the cylinder and a coolant passage. Confirm with a gauge before accepting it.

  6. A wiring or connector fault on the cylinder 4 coil or injector

    Worth a look before parts are bought, particularly on the rearmost cylinder where the harness has been disturbed for other work.

What to check, in order

  1. Note when the misfire happens, and whether the light flashes or stays steady.

    A misfire only when cold points at fuel delivery or a plug that fails to fire until things expand. A misfire only under load points at ignition breaking down under cylinder pressure. A misfire at idle that clears with revs points at a port leak. A flashing light means unburnt fuel is reaching the catalytic converter and driving on will damage it.

  2. Confirm which physical cylinder is number four on this engine.

    On an inline four it is the one furthest from the drive belts, usually against the bulkhead. On a V6 it is commonly the first cylinder of the second bank rather than anything you can count to from the front. Every test after this is worthless if the cylinder is wrong.

  3. Swap the coil from cylinder 4 to a neighbouring cylinder, clear the code and drive it.

    A misfire that moves with the coil is the coil. A misfire that stays on cylinder 4 clears the coil completely.

  4. Swap the spark plug the same way, and read the one you take out.

    A fault that follows the plug is the plug. A wet or oily plug points at fuel delivery or oil control rather than ignition.

  5. Compare injectors, then compression test cylinder 4 against its neighbours.

    Even injectors and even compression put the cause back on the intake side, so look for a leaking runner or port gasket. Compression low on cylinder 4 alone ends the electrical diagnosis.

Where this one turns up most

  • The bulkhead end of transverse inline four engines, where the coil sits in the hottest, least ventilated spot and is rarely replaced preventively.
  • Direct injection engines with carbon build up on the intake valves, which affects the cylinders unevenly.

Codes that often show up alongside it

What a DriveDecode adapter can read

A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.

Reading P0304 on your own car

DriveDecode pulls P0304 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.

See how DriveDecode works

Last reviewed 2026-09-08. Definition sourced from the published standard.