Everything you need to know about OBD2 fault codes, the AI-Diagnostics-Pro service and UK MOT testing.
OBD-II (On-Board Diagnostics II) is a standardised system that allows diagnostic devices to interface with a vehicle's computer. It has been mandatory in the UK for petrol cars since 2001 and diesel cars since 2004. The OBD-II port is a 16-pin connector, usually located under the dashboard on the driver's side within 2 feet of the steering wheel.
A fault code (also called a DTC — Diagnostic Trouble Code) is stored by your car's ECU (Engine Control Unit) when it detects a problem. Codes start with a letter (P = powertrain, B = body, C = chassis, U = network) followed by four digits. For example, P0420 indicates a catalytic converter efficiency problem.
You need an OBD-II scanner, which plugs into the 16-pin OBD port under your dashboard. Basic scanners cost from £8 on Amazon. Once plugged in, the scanner reads and displays any stored fault codes. You then enter the code into AI-Diagnostics-Pro along with your vehicle details to get a full UK diagnostic report.
An active check engine light (MIL — Malfunction Indicator Lamp) is an automatic MOT failure under current DVSA testing standards, regardless of the underlying fault. Even if the vehicle drives normally, if the warning light is illuminated when you arrive for your MOT, the test will fail. AI-Diagnostics-Pro includes MOT risk assessment in every report.
Basic fault code information is free on our /code/ pages. Full AI-powered UK-specific diagnostic reports — including vehicle-specific repair costs, step-by-step fix guides and MOT advice — are £1.59 per report. This is a one-time payment with no subscription required.
Yes. All cars sold in the UK since 1996 use the standardised OBD-II system, which means our service covers all major manufacturers including Ford, Vauxhall, BMW, Volkswagen, Toyota, Mercedes-Benz, Audi, Renault, Peugeot, Nissan, Honda, Hyundai, Kia, Land Rover, Volvo, Mazda, MINI, Fiat, Seat, Skoda, Dacia, MG, Tesla and more.
Yes. All repair cost estimates are based on current UK market data for parts and labour, including VAT, at average UK independent garage rates. We do not use US or European data. Costs will vary by region and vehicle — independent garages are typically 30–50% cheaper than franchised main dealers.
If the check engine light is flashing, stop driving immediately and arrange recovery — a flashing light indicates a serious fault such as a severe engine misfire that can cause immediate damage. If the light is solid and steady, it is usually safe to drive short distances, but you should get the code scanned as soon as possible.
Cars manufactured before 1996 may use OBD-I, which is not standardised. OBD-I protocols vary by manufacturer and require specialist equipment. AI-Diagnostics-Pro is designed for OBD-II vehicles (UK petrol from 2001, diesel from 2004, and most cars from 1996 onwards).
We do not store personally identifiable information. Vehicle data (year, make, model, engine, fault code) is used only to generate your specific report and is not retained after your session ends. See our Privacy Policy for full details.