How to read fault codes
Everything you need to know, from plugging in a reader to understanding what the code actually means.
Step by step
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1. Get an OBD-II reader
You'll need a code reader to pull the fault codes off the car. Basic ones start around £15, and Bluetooth ELM327 dongles work with a free phone app. Don't want to buy one? Plenty of garages and fast-fit centres, Halfords and Kwik Fit among them, will read the codes for free.
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2. Find the OBD port
Almost always tucked under the dashboard on the driver's side, within arm's reach of the wheel. A 16-pin trapezoid socket, sometimes hidden behind a small flip cover.
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3. Plug in with the ignition off
Push the reader into the port, then turn the ignition to position II (accessories on, engine off) or start the engine. Check your reader's instructions; they'll say which one it wants.
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4. Read the codes
Hit 'Read codes'. Anything stored or pending shows up on screen, formatted as P0300, C0021, B1001, that sort of thing.
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5. Look up the code here
Drop it into the search bar at the top of this page or the homepage. We'll spell out what it means, how serious it is, and what to do about it.
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6. Fix first, then clear
Repair the actual fault before you touch the reader's 'Clear codes' button. Clear it first and the light just comes back once the car re-checks itself.
Common questions
What is OBD and where is the port?
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OBD stands for On-Board Diagnostics. All petrol cars made after 2001 and diesel cars made after 2004 sold in the UK are required to have an OBD-II port. It's usually under the dashboard on the driver's side, within 60cm of the steering wheel. It looks like a 16-pin trapezoid socket.
What code reader do I need?
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For basic fault code reading, any generic OBD-II reader works, they start from around £15 online. Bluetooth dongles (ELM327-compatible) pair with a smartphone app. For manufacturer-specific codes and live data, you need a more capable reader, garages use tools like Autel, Launch, or Snap-on.
Can I clear the fault code myself?
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Yes, most code readers have a 'clear codes' function. This turns off the warning light and resets the code. However, it does not fix the underlying problem. The code will return once the car's systems run through their checks again, usually within a few drive cycles.
What's the difference between a generic and manufacturer-specific code?
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Generic OBD-II codes (P0xxx, C0xxx, B0xxx, U0xxx) are standardised across all makes and models, P0300 means the same thing in a Ford as it does in a BMW. Manufacturer-specific codes (P1xxx, P2xxx, C1xxx, B1xxx etc.) are defined by the vehicle manufacturer and can vary between brands for the same code number.
My warning light came on then went off, do I still need to check it?
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Yes. Intermittent faults still store a code in the car's memory even if the light clears itself. These are called 'pending' or 'stored' codes. An OBD reader will find them. Ignoring an intermittent fault often means it returns as a permanent one later.
Is it safe to drive with a warning light on?
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It depends entirely on the code. A minor emissions code or a sensor fault with no drivability impact is usually safe to drive on for a short period. An oil pressure warning, coolant temperature alarm, or any red warning light should be treated as stop immediately. When in doubt, stop and get it checked.