The OnBoard Diagnostics vehicle plug has 16 pins. Normally this female plug is only equipped with the metal pins that the car needs in order to satisfy the protocols that it’s Electronic Controller Units (ECUs) “speak”. Therefore, it is possible (within a reasonable limit),to “guess” which protocol(s) a car complies to, just by looking at those pins. The pins are numbered as follows: Examples (may be incorrect!): An ISO9141-2 car needs pins 5,7,16, and may need 15. An ISO14230-4 car needs pins 5,7,16, and may need 15. An SAE-J1850 (VPW) car needs pins 2,5,16. An SAE-J1850 (PWM) car needs pins 2,5,10,16. An ISO-15765-4 car needs pins 5,6,14,16. NOTE-1: It seems the L-line is “optional” for both ISO9141-2 and ISO14230-4, and that probably only older cars will need it… NOTE-2: It is possible that a car suports more than one “official” protocol, and so it will have a combination of these options. They normally will also sport a few of those “discretionary” pins… (there’s a lot to hack NOTE-3: Be advised that ALL the pins (except 4,5,16) can have non-standard allocation!!! This means that (for example) an ISO9141-2/ISO14230-4 car can use the SAE-J1850 pins for whatever they want (and vice-versa)!!!! The only rule stated in the specs is that when this happens, those pins cannot confuse or damage a specification-compliant tool!… NOTE-4: You may or may not use pin 4 (chassis ground). If you draw power from the plug, then it’s a good idea to use it (max power = 12Vx4A = 48W).