Cylinder 4 misfire on Vauxhall — 1.4/1.6 Turbo coil failure, 1.9/2.0 CDTi injector or glow plug, older 1.6/1.8 plug HT lead issues.
Vauxhall P0304 — Cylinder 4 misfire on Vauxhall — 1.4/1.6 Turbo coil failure, 1.9/2.0 CDTi injector or glow plug, older 1.6/1.8 plug HT lead issues. Typical UK independent garage cost: £100–£480.
What actually causes P0304 on a Vauxhall
Vauxhall 1.4T A14NET coil packs fail consistently around 60k miles — cyl 4 first due to heat. Insignia 2.0 CDTi cyl 4 misfire is usually injector wiring loom chafe (same loom that causes P0203). Older 1.8 16v Astras (Z18XER engine) suffer carbon deposits on intake valves causing cyl 4 misfire — walnut blast clears it.
Vauxhall engines that log P0304
- 1.4 Turbo (Astra J/K, Mokka, Insignia)
- 1.6 Turbo (Astra J VXR, Insignia)
- 1.9/2.0 CDTi (Vectra C, Astra H/J, Insignia)
- 1.6/1.8 16v (older Astra G/H)
How we diagnose P0304 on a Vauxhall
- Tech2 / OP-COM — read per-cylinder misfire counters
- Swap coil pack 4 with another cylinder
- Pull plugs, inspect
- On CDTi, inspect injector wiring loom for chafe at alternator bracket
- Replace component, road-test
The common misdiagnosis to avoid
1.4T Astra/Mokka with P0304 is so often a coil that some indies just replace cyl 4 coil on first visit and only investigate further if it returns — saves diagnostic time and the customer's happy.
Vauxhall P0304 repair cost in the UK
Expect £100–£480 including parts and labour at an independent garage. Main-dealer quotes on Vauxhall are usually higher for the same job, and the spread exists because the cheap cause and the expensive cause throw the same code — pay for the diagnosis before you pay for parts.
Bob's workshop tip
On Insignia 2.0 CDTi, when you see P0304 alone or with P0204, ALWAYS strip and inspect the injector loom on the rear bank — chafed wiring is the cause maybe 60% of the time and a £20 repair instead of £400 injectors.
Codes usually seen alongside P0304 on Vauxhall
- P0300
- P0301
- P0302
- P0303
- P0204
Symptoms drivers report with P0304
- 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
Repairs that clear P0304
- Plug & coil replacement
For the generic, all-makes explanation of this code — meaning, every known cause and UK cost bands — see our P0304 guide.
Reading P0304 on a Vauxhall properly
A generic reader will show P0304 and nothing else. What matters is the freeze-frame data stored with it — engine speed, load, coolant temperature and fuel trims at the moment the fault was logged — plus whether any other module on the Vauxhall has logged something at the same time. Codes rarely arrive alone, and the order they were stored in usually points at which one is the cause and which are the consequences.
Clearing P0304 without fixing it
Erasing the code resets the Vauxhall's readiness monitors as well as the light. The fault returns on the next drive cycle if the cause is still there, and in the meantime the incomplete monitors can stop the car passing an emissions check. After a genuine repair, the car is driven until the relevant monitor completes before it is handed back.
Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P0304 is showing on your Vauxhall, bring any scan results with you — we will read the live data ourselves, confirm which cause it actually is, and price the repair before any work starts.