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The OBD2 protocols: how your car talks

The socket is the same on every car, but the electrical language running through it isn’t. The standard allows five, and the adapter has to find the one your car uses.

ProtocolSpeedPinsWho uses it
CAN (ISO 15765-4)250 or 500 kbit/s6 and 14Nearly every car since the mid 2000s. In the United States, mandatory from the 2008 model year.
KWP2000 (ISO 14230-4)10.4 kbit/s7 (sometimes 15 as well)Many European and Asian cars from roughly 2000 to 2008.
ISO 9141-210.4 kbit/s7 (sometimes 15 as well)European, Asian and Chrysler cars from the late nineties and early two thousands.
SAE J1850 VPW10.4 kbit/s2US General Motors, before CAN.
SAE J1850 PWM41.6 kbit/s2 and 10US Ford, before CAN.

CAN, the one nearly everyone uses

CAN is a fast two wire network, CAN high and CAN low, over which the control units talk to each other and which the scanner joins to ask questions. It’s what you’ll find on any car from well over the last fifteen years. It has four variants depending on speed (250 or 500 kbit/s) and address length (11 or 29 bits); most European cars run at 500 kbit/s with 11 bit addresses.

Over CAN, long answers travel split into several frames in a row, and that’s where clone adapters choke: it’s all explained in the adapter guide.

The K-line: ISO 9141-2 and KWP2000

Before CAN, most European cars talked over a single wire, the K-line, on pin 7. It’s slow and very strict about timing: to get started the control unit has to be woken up with a special greeting, either at 5 baud (a single byte that takes two seconds to send) or with the KWP2000 fast init. If the adapter doesn’t hit those timings exactly, it won’t connect.

Besides OBD, many makes used the K-line for their own diagnostic protocols, like KW1281 on nineties Volkswagens, which an ordinary OBD2 adapter can’t always speak.

J1850, an American thing

The two J1850 variants, General Motors’ VPW and Ford’s PWM, are only found on cars built for the United States before 2008. They’re rare in Europe, and many cheap adapters don’t even support them.

How the adapter knows which one it is

With the ignition on, the adapter tries the protocols one by one until the car answers, and the first time it takes a few seconds. Final OBD detects which one it is when it connects and remembers it, so next time it starts with the right one.

And after CAN

Newer cars add faster networks between their control units, like CAN FD or ethernet, and some are diagnosed at the garage over ethernet (DoIP). Emissions OBD is still read through the same socket, and an OBD2 adapter is still fine for the basics. What gets requested over these protocols is in the OBD modes.