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How to read and clear your car’s fault codes

A warning light comes on and the first thing is to know what’s going on. With an OBD2 adapter and your phone you know in a couple of minutes, without a trip to the garage just for that.

In short: plug in the adapter, switch the ignition on (no need to start the engine), and the app asks the car for its faults and explains them. Clearing is asking it to forget them, but if whatever caused them is still there, they come back. So read, note them down and fix first; clear afterwards. Standard OBD shows the engine ones; for every control unit you need an app that talks to each of them.

Open the bonnetthe in-depth explanation

Step by step

  1. Plug the OBD2 adapter in under the steering wheel.
  2. Switch the ignition on. You don’t need to start the engine to read.
  3. Open Final OBD and pick the adapter. The first time takes a little longer; after that it remembers.
  4. Go to OBD faults for the engine ones through standard OBD, which you read and clear for free with Free, or to Advanced faults for every control unit, one by one, which comes with Pro.
  5. Look at each code with its explanation. In Advanced faults, with the full explanation and what to check.

What each code means

OBD2 codes have a letter and four digits. The letter tells you where the fault comes from:

  • P, powertrain: engine and gearbox. It’s the one you’ll see most.
  • C, chassis: brakes, ABS, steering and suspension.
  • B, body: airbag, locking, climate control and lights.
  • U, communication between control units.

If the first digit is 0, the code is generic and means the same on every make: P0234, for example, is turbo overboost on any car. If it’s 1, the manufacturer defined it, and that’s where an app that knows your make makes a difference.

Confirmed, pending or permanent

Not all faults carry the same weight, and Final OBD keeps them apart:

  • Confirmed: the car has seen the fault enough times and usually turns the warning light on. It needs looking at.
  • Pending: it has seen it once and is waiting to confirm it. It may come to nothing or get worse.
  • Permanent: it can’t be cleared. It goes away on its own when the car checks, while driving, that the fault is fixed.
  • Anything that only failed once in a while is shown separately, so it doesn’t worry you.

How the car was running when it happened

Many control units also store how the engine was running when the fault was logged: revs, speed, temperature or load. It’s very useful, because a turbo fault at 4,000 rpm climbing a hill is not the same as one from cold just after starting, but Final OBD doesn’t show that data yet.

When to clear, and when not to

Clearing a fault doesn’t fix anything: it only switches the warning off. If the fault is still there, it comes back. Clear it once it’s fixed, or to see whether it returns.

  • Do it with the car stationary, the ignition on and the battery in good health.
  • After clearing, the car restarts its emissions checks from scratch and takes a few days of driving to finish them.
  • Don’t clear just before a roadworthiness test: a recent clear shows. It’s covered in the ITV and OBD guide.
  • On some recent cars the manufacturer adds a security gateway: everything can be read, but it may not let you clear.

Reading changes nothing in the car. Clearing faults writes to the control units. Check the result shown by the app.

Which version you need

With FREE you read the engine faults through standard OBD for nothing, each with its code and its explanation, and clear them. With PRO, you read the faults of every control unit, each with its explanation, and clear them on every one; what it costs is on the prices page. The first time, you get a free PRO trial. What each code means is in the list of fault codes. See what there is for your make in the list of compatible cars.