The OBD port and scan tools

What OBD-II is

OBD stands for on-board diagnostics. The engine computer watches its own sensors and stores a code when something is wrong. OBD-II is the common standard that lets one tool talk to cars of every make. It has been required on new cars in the United States since 1996 and in the European Union since 2001 for petrol cars and 2004 for diesels. In India, on-board diagnostics came in with the Bharat Stage IV rules, in steps from 2010, and the rules were tightened again with Bharat Stage VI.

Where the socket is

The diagnostic socket has 16 pins in two rows and is shaped like a flattened D. It is inside the cabin, almost always under the dashboard on the driver's side, near the steering column or above the pedals. In some cars it hides behind a small cover, near the fuse box or in the centre console. The owner's manual shows the place.

Three kinds of tool

• Basic code reader. A small hand-held unit with a cable. It reads and clears engine codes and shows whether the emission self-tests are complete. Enough for most owners.
• Bluetooth or Wi-Fi adapter. A small plug that sends data to an app on your phone. Besides codes, the app can show live values such as coolant temperature and fuel trim. Quality varies a lot between adapters.
• Full scan tool. Workshop equipment that talks to every computer in the car (gearbox, ABS, airbags, body), runs tests and does coding. This is what a mechanic uses to find the cause.

How to read codes

1. Switch the ignition off and plug the tool into the socket.
2. Switch the ignition on. You do not need to start the engine.
3. Choose "read codes". Write down every code, and whether it is shown as stored, pending or permanent.
4. If the tool offers "freeze frame", note that too: it shows the engine conditions at the moment the fault was recorded.
5. Look the codes up before clearing anything.

Good habits

• Do not leave a cheap adapter plugged in for days: some drain the battery.
• Do not clear codes just before taking the car to a workshop. The stored information helps the mechanic.
• Never use a phone app while driving. Ask a passenger, or record the data and look afterwards.