What Communications Protocol does my vehicle use?

As a rule of thumb, GM cars and light trucks use SAE J1850 VPW (Variable Pulse Width Modulation). Chrysler products and all European and most Asian imports use ISO 9141 circuitry. Fords use SAE J1850 PWM (Pulse Width Modulation) communication patterns.

There are some variations among captive imports such as the Cadillac Catera, a German Opel derivative, which uses the European ISO 9141 protocol.

On 1996 and later vehicles, you can tell which protocol is used by examining the OBD II connector:

J1850 VPW–The connector should have metallic contacts in pins 2, 4, 5, and 16, but not 10.
ISO 9141-2–The connector should have metallic contacts in pins 4, 5, 7, 15, and 16.
J1850 PWM–The connector should have metallic contacts in pins 2, 4, 5, 10, and 16.
J1939 Can Bus–The connector should have metallic contacts in pins 4,5,6,14, and 16.

If your vehicle has this style connector, but doesn’t have these pins populated, you probably have a pre-OBDII vehicle. To add some confusion, even having the connector with the contacts shown above is not a guarantee of OBD II compliance. This style connector has been seen on some pre-1996 vehicles which were not OBD II compliant.