P0304 indicates a misfire on cylinder 4. UK repair costs, causes and fixes for OBD2 fault code P0304.
P0304 indicates a misfire on cylinder 4. UK repair costs, causes and fixes for OBD2 fault code P0304. When this fault is detected, the ECU (Engine Control Unit) stores the code and illuminates the check engine light (also called the engine management light or malfunction indicator lamp) on your dashboard.
All cars sold in the UK since 1996 use the standardised OBD-II diagnostic system, meaning fault code P0304 can affect any manufacturer — including Ford, Vauxhall, BMW, Volkswagen, Toyota, Mercedes-Benz, Audi, Renault, Peugeot, Nissan, Honda, Hyundai, Kia, and all others. The code is read using an OBD-II scanner plugged into the 16-pin diagnostic port, usually located under the dashboard on the driver's side.
These are the most common symptoms UK drivers report when this code is active:
Note that some vehicles show no drivability symptoms at all — the check engine light may be the only sign. This is why scanning the fault code is essential rather than guessing at the cause.
Listed from most to least common based on UK garage data:
It is important to diagnose the root cause before replacing parts. Many unnecessary and expensive repairs happen because the most expensive component is replaced first without proper testing. Always work through the diagnostic steps systematically.
Work through these steps in order to avoid unnecessary parts replacement:
After any repair, clear the fault code with an OBD-II scanner and carry out a test drive. If the code returns, continue to the next diagnostic step. For a personalised repair guide specific to your vehicle's make, model, year and engine, use our AI Diagnostic Report below.
These models are frequently seen with this fault code in UK garages:
This code can appear on any OBD-II vehicle (all UK cars since 1996) but is more common on the models listed above. If your vehicle is not listed, it does not mean the code cannot affect it — it simply means these models are statistically more frequent in UK repair data.
Difficulty: Easy — coil and plug replacement DIY.
UK repair cost estimate: £60–£400 including parts and labour at average UK workshop rates (inc. VAT). Independent garages are typically 30–50% cheaper than main dealers. Always get at least two quotes.
Prevention: Regular spark plug replacement on schedule.
Fault code P0304 is likely to cause an MOT failure. Under current DVSA testing standards, an illuminated malfunction indicator lamp (MIL / check engine light) is an automatic MOT failure, regardless of whether the vehicle is otherwise roadworthy. This means you must resolve this fault before your MOT test.
Even if the light is not currently on, if the fault is intermittent it could reappear during the test. Use our AI Diagnostic Report to understand the MOT risk specific to your vehicle and get advice on whether it is safe to proceed to your test.
Specific to your vehicle — year, make, model and engine.
Includes step-by-step repair guide, exact UK labour costs and MOT impact.
Can I drive with fault code P0304?
Avoid driving — cylinder misfires can cause catalytic converter damage quickly.
Will P0304 clear itself?
Some fault codes will clear automatically after the underlying issue resolves itself, but P0304 typically stays stored until the fault is fixed and the code is manually cleared with an OBD-II scanner. Do not assume the problem has gone away if the light turns off temporarily.
Do I need a garage or can I fix P0304 myself?
Easy — coil and plug replacement DIY.
How much does it cost to fix P0304 in the UK?
UK repair costs for P0304 typically range from £60 to £400 depending on your vehicle, the root cause, and your local garage rates. Use our AI Diagnostic Report for a cost estimate specific to your car.
These fault codes are commonly found alongside P0304 or share similar causes:
Last updated: May 2026. AI-Diagnostics-Pro provides AI-generated information for educational purposes only. It is not a substitute for professional automotive advice. Always consult a qualified mechanic before carrying out vehicle repairs. Repair cost estimates are approximate and may vary by region and vehicle.