Reading fault codes with a cheap OBD2 scanner

Written by

A warning light on the dash used to mean a trip to the garage just to find out what it meant. Now a €15 adapter and your phone can tell you, and often point you to a fix you can do yourself.

What is OBD2?

Every petrol car sold in the EU since 2001, and diesel since 2004, has an OBD2 (on-board diagnostics) port. It’s a 16-pin socket, usually under the dashboard near the steering column. Plug a reader in and you can see:

Fault codes stored by the engine computer, like P0401
Live data: coolant temperature, boost pressure, fuel trims, sensor readings and more
Readiness monitors, which show whether the car’s self-tests have completed

What you need

An ELM327-style adapter (Bluetooth or Wi-Fi). They’re cheap, but quality varies a lot. Cheap clones can be unreliable, so buy one with good reviews.
An app: for example Car Scanner ELM OBD2 or Torque Pro on Android, or Car Scanner on iPhone.

Reading a code, step by step

Plug the adapter into the OBD2 port, and turn the ignition on (engine off or running).
Connect the app to the adapter and run a scan.
Write down every code, plus the freeze frame data if the app shows it: a snapshot of the conditions when the fault happened.
Look the code up for your specific car and engine. Owner forums are gold here.
Fix the cause, then clear the code and check that it doesn’t come back.

Example: P0401 on a diesel

P0401 means “EGR flow insufficient”. It’s common on diesels, where the exhaust gas recirculation valve and pipes clog with soot. The code doesn’t mean “replace the EGR valve”; it means the computer saw less flow than expected. Cleaning the valve, checking the pipes and vacuum lines, or a long run might sort it. This is why you research the code rather than just throwing parts at it.

The limits of a cheap scanner

Generic OBD2 mostly covers engine and emissions faults. ABS, airbags, the electronic parking brake, DPF regeneration and service resets usually need manufacturer-level software:

Citroën and Peugeot (like my C4 Grand Picasso): Diagbox with a compatible interface
Volkswagen Group (like the A4 I had): VCDS or OBDeleven
Ford (like the S-Max I had): FORScan, which works with some modified ELM327 adapters

These can read every module in the car, run tests and do jobs like coding or forcing a DPF regeneration.

Don’t just clear codes

Warning Clearing a code turns the light off, but it doesn’t fix anything, and the light will come back. Clearing codes also resets the readiness monitors, which is a giveaway that something’s been hidden. Use the scanner to understand the problem, not to hide it.