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:

  ____________________________________
  \  1   2   3   4   5   6   7   8   /  (Seen from the side of
   \ 9   10  11  12  13  14  15  16 /    the connector where
    \______________________________/     you push in the plug)
 
 The following information was extracted from a few specifications:
 (format is: Pin number - Pin usage)

  1 - Discretionary;
  2 - SAE-J1850 (VPW/PWM) positive bus line;
  3 - Discretionary (airbag|...);
  4 - Chassis ground (battery negative);
  5 - Signal ground (max 1.5 Amp);
  6 - ISO-15765-4 CAN_high line;
  7 - K-line of ISO-9141-2 and ISO-14230-4 (data line);
  8 - Discretionary (Code|...);

  9 - Discretionary;
 10 - SAE-J1850 (PWM) negative bus line;
 11 - Discretionary(alarm|remote|...);
 12 - Discretionary;
 13 - Discretionary;
 14 - ISO-15765-4 CAN_low line;
 15 - L-line of ISO-9141-2 and ISO-14230-4 (init only);
 16 - Positive voltage (battery positive, max 4 Amp).

 "Discretionary" means that the car manufacturer can use the pin
 for whatever they want (so if it's equipped, it's a "proprietary" pin).

 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).

Enjoy!