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OBD2 connector
Every car and light truck sold in the US since the 1996 model year has the same 16-pin diagnostic connector, defined by SAE J1962. Here is what each pin does and which ones your car actually uses.
The connector
Viewed from the front of the car's socket. Highlighted pins are the ones the standard defines; the rest are left to each manufacturer.
1
Maker
2
J1850 +
3
Maker
4
Chassis ground
5
Signal ground
6
CAN high
7
K-line
8
Maker
9
Maker
10
J1850 -
11
Maker
12
Maker
13
Maker
14
CAN low
15
L-line
16
Battery +
Pin by pin
Pins 16, 4, 5
Pin 16 carries unswitched battery voltage, which is what powers a plug-in code reader. Pin 4 is chassis ground and pin 5 is signal ground.
Pins 6, 14
CAN high on pin 6 and CAN low on pin 14 (ISO 15765-4). CAN has been required on US vehicles since the 2008 model year and is what most modern scan tools talk over.
Pins 7, 15
Used by ISO 9141-2 and ISO 14230-4 (KWP2000), common on many European and Asian vehicles before CAN. Pin 15 is optional and often empty.
Pins 2, 10
Older domestic protocols. J1850 VPW uses pin 2 only, while J1850 PWM uses pins 2 and 10.
Pins 1, 3, 8, 9, 11, 12, 13
Left to each manufacturer for its own diagnostics and programming. What they do varies by make, model and year, so check your car's workshop manual.
Where is it?
The port must be within reach of the driver, usually under the dashboard near the steering column. Some cars hide it behind a small trim panel.
Plug in a reader, note the code, then see what it means and how urgent it is.