How to read and clear your car’s fault codes

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 Faults. With Free you read the engine ones; with Pro, every control unit, one by one.
5. Look at each code with its explanation and, where there is one, how the car was running when it happened.

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 store a snapshot of the moment of the fault: revs, speed, temperature or engine load. Final OBD shows it next to each fault, with Pro. A turbo fault at 4,000 rpm climbing a hill is not the same as one from cold just after starting, and that helps a lot to understand what happened.

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, service functions and adaptations do write to the control units, which is why the app reads the value before and reads it again afterwards to check the change went in.

Which version you need

With Free you read the engine faults, explained, for nothing. With Pro, every control unit, how the car was running when each fault happened and clearing them, for €24.99 once. See what there is for your car in makes and models.