P0300 means the engine has detected random or multiple-cylinder misfires. In plain English, combustion is going wrong often enough for the ECU to notice, but not neatly enough to pin it to one cylinder. The repair cost in the UK depends on whether the issue is a simple ignition fault or something deeper like fueling, compression, or timing.
P0300 repair cost in the UK can be modest if the cause is plugs, coils, or a basic air leak, but it rises quickly if the misfire turns out to be fueling, compression, or timing related. Because an active misfire can damage the catalytic converter fast, urgency matters as much as cost.
This guide uses standard misfire-diagnosis workflow and common UK repair-cost bands to show why P0300 should not be treated as a single fixed-price fault. It is intended to help you question quotes intelligently, not replace hands-on testing. For how we explain diagnostics and review tools, see our editorial approach, customer reviews, and scanner recommendations.
| Likely cause | Typical UK cost | Comment |
|---|---|---|
| Spark plugs | £60–£150 | Often the cheapest starting point |
| Ignition coil pack or coils | £80–£220 | Common on petrol engines |
| Injector or fueling issue | £120–£350 | More testing needed before parts |
| Vacuum leak or intake issue | £70–£220 | Can mimic ignition faults |
| Compression or mechanical issue | £250–£1,200+ | Higher-risk outcome if basic fixes fail |
Very likely. A misfire usually leaves the engine management light on and pushes emissions in the wrong direction at the same time. If you have P0300, treat MOT failure risk as high until the misfire is repaired and the light stays off.
Only with caution if the light is solid and the engine still runs reasonably well. If the light is flashing, the engine is running badly, or the car is down on power, stop driving. The longer you keep driving a misfiring car, the more likely you are to turn a modest ignition fix into a catalytic-converter bill.
Use your code and vehicle details to see likely causes, urgency, and realistic UK repair costs before spending money at a garage.
Get My AI Diagnostic Report — £1.59 →Disclaimer: Informational guidance only. Persistent or flashing misfires should be diagnosed quickly to avoid catalyst damage.