In 1970 The United States Congress passed the Clean Air Act to help reduce the worlds smog problems. Car manufacturers started to introduce electronic components in to carburetors in a bid to achieve the lower emission standards and maintenance free ignition systems were entering mass production.

Diagnostic Link Connectors (DLC's) were being introduced to vehicles so sophisticated test equipment could be connected. This enabled a mechanic to monitor the running of an engine and also to test for faults.

In 1976 the lambda sensor was born and tailpipe emissions were coming down.

In 1978 the first Engine management system was born. It was the Bosch Motronic and as the name states it was developed by Bosh. It was introduced by BMW and the new electronic era was born.

Drivers now had this strange addition of a new warning lamp in the instrument panel. It brought on a whole new world of doubt as to why this light was there and what it really meant if it came on. On Board Diagnostics was now mainstream.

In 1985 OBD-1 was introduced to standardise the way in which vehicle systems could be monitored. The list of functions OBD1 would have to achieve grew with the need for real time misfire detection, Lambda sensors, the catalytic converter and fault code capability.

Next came OBD-1.5. This was a partial introduction to OBD11, a beta version if you like. General motors used it in some 1994 and 95 vehicles. Post catalytic lambda sensors were being fitted and the standards that dictated OBD11 brought us the P Code. (Trouble Codes)

In 1996 OBD-11 was introduced and with this come the 16 pin Diagnostic connector that we see today. The OBD-II standard specified the type of diagnostic connector,the messaging format and the way in which the electrical signaling protocols would be wired to it. The OBD-II standard also provided an extensive list of Diagnostic Trouble codes (DTC's) As a result of this standardization, a single device could query the on-board computers in any vehicle.
 

OBD-II 16 Pin Diagnostic connector

The OBD-II specification provides for a standardized connector.,The female 16-pin J1962 connector. Unlike the OBD-I connector, which was sometimes found under the bonnet of the vehicle, the OBD-II connector is nearly always located on the driver's side  of the car often close to the steering column. SAE J1962 defines the pin out of the connector as:

1. -
2. Bus positive Line of SAE-J1850
3. Ford DCL(+) Argentina, Brazil (pre OBD-II) 1997-2000
4. Chassis ground
5. Signal ground
6. CAN high (ISO 15765-4 and J2234)
7. K line of ISO 9141-2 and ISO 14230-4
8. -
9. -
10. Bus negative Line of J1850
11. Ford DCL(-) Argentina, Brazil (pre OBD-II) 1997-2000
12. -
13. -
14. CAN low (ISO 15765-4 and J2234)
15. L line of ISO 9141-2 and ISO 14230-4
16. Battery voltage

Unspecified pins are left to the vehicle manufacturer's discretion which can be connected up to model specific systems.

Today systems are more complicated than ever and there is a plethora of Diagnostic equipment both OEM and aftermarket. They all vary in ability and you should choose wisely when purchasing your diagnostic tool.