P0304 fault code: what it means

P0304: Cylinder 4 is misfiring. Swapping the coil to another cylinder shows whether the coil is the cause.

Engine and gearbox (powertrain) · Costly if missed

Usual causes, most likely first

failed ignition coil on cylinder 4
worn or fouled spark plug
clogged or leaking injector
low compression (valve or ring wear)

P0304 by make

Audi

Cylinder 4 misfire. On TSI/TFSI swap the coil to another cylinder first; if the misfire stays put, look at plug, injector, carbon or compression.

Usual causes: failed ignition coil; worn spark plug; carbon on intake valve (direct injection); injector fault or low compression

BMW

Cylinder 4 misfire.

Usual causes: ignition coil; spark plug; fuel injector (DI engines); low compression

Honda

Misfire on cylinder 4. On Honda V6 with VCM, the deactivated cylinders often foul plugs from oil burning.

Usual causes: fouled or worn spark plug (VCM oil fouling on V6); ignition coil; tight valve clearance; low compression or stuck piston rings

Jeep

Cylinder 4 misfire. Same left-head pattern as cylinders 2 and 6 on early 3.6 Pentastar.

Usual causes: spark plug or coil on cylinder 4; left cylinder head valve problem (early Pentastar); injector fault

Lexus

Cylinder 4 misfire. On IS250/GS300 direct-injection V6 engines single-cylinder misfires at warm idle are a classic sign of intake valve carbon.

Usual causes: carbon build-up on intake valves (walnut blast clean); failing ignition coil; worn spark plug

Mazda

Misfire on cylinder 4. On Mazda petrols swap the coil to another cylinder to confirm; plug and coil are the usual fix, then injector.

Usual causes: ignition coil on that cylinder; spark plug; fuel injector on that cylinder; low compression on that cylinder

Mercedes-Benz

Misfire detected in cylinder 4. On Mercedes petrols usually a spark plug or ignition coil on that cylinder.

Usual causes: worn spark plug; failed ignition coil; faulty fuel injector

Mini

Cylinder 4 misfire.

Usual causes: ignition coil; spark plug; fuel injector; low compression

Skoda

Cylinder 4 misfire. On TSI/TFSI swap the coil to another cylinder first; if the misfire stays put, look at plug, injector, carbon or compression.

Usual causes: failed ignition coil; worn spark plug; carbon on intake valve (direct injection); injector fault or low compression

Subaru

Misfire on cylinder 4. Swap the coil to another cylinder to see if the misfire follows it.

Usual causes: worn spark plug or failed ignition coil; oil in the spark plug tube from leaking valve cover/tube seals (EJ25); EJ25 head gasket leak fouling the plug; leaking or blocked injector

Volkswagen

Cylinder 4 misfire. On TSI/TFSI swap the coil to another cylinder first; if the misfire stays put, look at plug, injector, carbon or compression.

Usual causes: failed ignition coil; worn spark plug; carbon on intake valve (direct injection); injector fault or low compression

Volvo

Cylinder 4 misfire.

Usual causes: ignition coil cylinder 4; spark plug; injector

What the repair usually costs

Head gasket replacement (4-cylinder)NZ$1,830–NZ$4,090 · A$1,770–A$4,010
Head gasket replacement (V6 / flat-4 / twin-head engine)NZ$3,860–NZ$7,330 · A$3,840–A$7,170
Head gasket replacement (diesel ute, 4-cylinder)NZ$3,300–NZ$6,340 · A$3,320–A$6,210
Petrol fuel injectors (port injection, set)NZ$430–NZ$1,420 · A$400–A$1,360

Not sure what P0304 means on your car? Ask Credov.

Before you clear it

Write the code down or photograph the scanner first. A code that comes straight back after clearing points to a fault that is still there. A car whose codes were cleared just before you arrived may be hiding one: the scanner's readiness monitors show "not ready" for a while after codes are cleared.

Researched by Credov from manufacturer and workshop sources. A fault code points to where a fault may be; a workshop confirms it with a diagnosis. Browse all fault codes.