Best OBD2 Scanners (UK) & How to Read Fault Codes
An engine warning light is one of the most stressful things on a dashboard — partly because it tells you so little. An OBD2 scanner changes that. For the price of a single garage diagnostic fee, you can read the exact fault code yourself, understand what's going on, and decide whether it's a five-minute fix or a trip to the workshop. Here's how to choose one and use it.
What OBD2 actually does
Every car sold in the UK since around 2001 (petrol) and 2004 (diesel) has an OBD2 port — usually under the dashboard near the steering column. Plug a scanner in and it talks to your car's computer, reading Diagnostic Trouble Codes (DTCs) and live sensor data. When the engine light comes on, the car has stored a code explaining why. The scanner reads it, and once you've fixed the issue, it can clear the code and reset the light.
Choosing the right scanner
There's a scanner for every budget. The right one depends on how deep you want to go:
- Basic code readers read and clear generic engine codes. Perfect if you just want to know why the light is on.
- Bluetooth dongles plug into the port and pair with a phone app, giving live data and graphs on your screen — great value and very portable.
- Mid-range handheld units add ABS, airbag and manufacturer-specific codes, plus features like live data logging.
- Advanced bi-directional tools can actuate components and perform resets — more than most owners need, but handy for keen DIYers.
Browse our range of OBD2 scanners to match the right tool to your car and your confidence level. Pair it with the right cables and adapters from our electronics accessories so it's ready when you need it.
How to read a fault code
- Switch the ignition on (engine off, or running if your tool needs it).
- Plug the scanner into the OBD2 port.
- Select "Read Codes" and note down every code shown.
- Look up each code — many scanners and apps explain them on screen.
- Diagnose and repair, then use "Clear Codes" to reset the light.
One word of caution: clearing a code doesn't fix the fault. If the underlying problem is still there, the light will return — and clearing codes just before an MOT emissions check won't help, as the car needs to complete its readiness monitors first.
Understanding the codes
Codes follow a pattern. The first letter shows the system: P for powertrain (engine and transmission), B for body, C for chassis, U for network. The numbers narrow it down.
Common P-codes you'll meet
- P0300–P0308: misfire detected (P0301 means cylinder 1). Often spark plugs or ignition coils.
- P0420 / P0430: catalytic converter efficiency below threshold — sometimes the cat, sometimes an oxygen sensor.
- P0171 / P0174: system running too lean — air leaks, a dirty MAF sensor or a vacuum leak.
- P0128: coolant temperature below thermostat regulating temperature — usually a stuck-open thermostat.
- P0455: large EVAP leak — frequently a loose or faulty fuel cap.
A misfire code pointing at a specific cylinder, for example, often leads straight to a worn spark plug or a failing coil — both straightforward DIY jobs once you know which one to blame.
What you'll need
- An OBD2 scanner suited to your car
- A torch to find the port under the dash
- A note of your car's make, model and engine
When the code points to a specific component, fitting the correct OE-matched part is what gets the light to stay off. Search by your car's OE part number for an exact fit, and if you're chasing a sensor or module for an older car that's hard to find, our request-a-part service can help you source discontinued and obscure parts — just send the part number. Owning a scanner pays for itself the first time it saves you a needless diagnostic bill.
Frequently asked questions
- Where is the OBD2 port on my car?
- On almost all UK cars it's within reach of the driver, usually under the dashboard near the steering column. Check behind a small cover or trim panel if you can't see it straight away.
- Will clearing the fault code fix the problem?
- No. Clearing the code only resets the warning light. If the underlying fault remains, the light will come back. Diagnose and repair the cause first, then clear the code.
- Do I need an expensive scanner?
- Not usually. A basic reader or a Bluetooth dongle paired with a phone app reads and clears engine codes for most owners' needs. Pricier tools add ABS, airbag and manufacturer-specific functions.