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Before OBD-I, each manufacturer had their own set of standards for OBD, meaning that mechanics had to buy expensive scan tools for each manufacturer. OBD-I was first introduced in 1987, and started the standardization of onboard diagnostics.

It had sensors that detected emissions and was able to minimize them through emissions-controlling valves. However, it had many problems and shortfalls.


As a result, in 1996 car manufacturers started to equip cars and trucks with an OBD-II port. Every system is mostly the same, but there are slight variations. These are known as protocols, and are specific to vehicle manufacturers.

There are five basic signal protocols:

SAE J1850 PWM: Pulse Width Modulation, used in Ford vehicles

SAE J1850 VPW: Variable Pulse Width used in General Motors vehicles

ISO9141-2: Used in all Chrysler and a variety of European or Asian vehicles

ISO14230-4 (KWP2000): Keyword Protocol, used in a variety of European and Asian imports as well as Honda, Jeep, Land Rover, Subaru, Mazda, Nissan, and more

ISO 15765 CAN: Controller Area Network, used on all vehicles manufactured after 2008

Pins 4 and 5 in all protocols are used for ground connections, and pin 16 is used for power from the car’s battery.

Once the computer senses a problem with the engine or any other component of the car it’s monitoring, it’ll trigger the Check Engine light. Some vehicles also blink the engine light if the problem is a very serious one.

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