Cylinder 4 misfire.
P0304 — also written PO3O4, P 0304, 0304 — is the OBD-II code for cylinder 4 misfire detected. Same code, same fix: this page covers every spelling of it.
Cylinder 4 misfire — on UK petrol cars, the usual suspects in order are coil pack, spark plug, then injector. Diesel misfires on cyl 4 are different and almost always injector or compression related.
P0304 is a ignition fault code, typically costing £30–£800 to put right in a UK garage.
What P0304 actually feels like to drive
- Engine warning light flashing (not steady) under load
- Noticeable juddering at idle in gear
- Smell of unburnt fuel from the exhaust
- Misfire worse when engine is cold and damp
- Significantly reduced power
What causes P0304?
- Spark plug
- Coil
- Injector
UK vehicles that throw P0304 most often
- Vauxhall Corsa / Astra 1.4 / 1.6 petrol — coil-on-plug failures
- Ford Fiesta / Focus 1.0 EcoBoost — wet belt and coil issues
- Mini Cooper N12/N14 petrol — coil pack and timing chain
- VW Polo / Golf 1.4 TSI — injector and coil
- Renault Clio 1.2 / 1.6
How P0304 is fixed
- Plug & coil replacement
The P0304 misdiagnosis that wastes money
A flashing MIL is serious — keep driving and you'll wreck the catalytic converter (£400–£900 replacement). The cheap mistake is replacing all four plugs and coils without first swapping the cyl 4 coil to another cylinder to see if the misfire follows it. 90 seconds of work confirms the diagnosis.
What a UK garage should charge for P0304
Single ignition coil £45–£140 fitted, spark plug set £35–£90 fitted, injector £180–£420 fitted, compression test £40–£70, cat replacement if damaged £400–£900.
P0304 repair cost in the UK
Most P0304 repairs land between £30–£800 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0304
If the misfire is intermittent and gets worse in the rain or after a car wash, it's almost always a cracked spark plug boot or coil pack letting moisture in. Five minutes with a torch and a spray bottle of water on a running engine will show you which one is arcing.
Can you drive with P0304?
Drive gently.
Urgency: high.