When Onboard Diagnostics II was first conceived as a means of monitoring vehicle emissions, it was obvious there would be a lot of information to keep track of. The Society of Automotive Engineers (SAE) developed guidelines for managing all of this information and came up with a list of "diagnostic modes" that scan tools would have to provide. The scan tool modes and other OBD II requirements can be found in a document called SAE J1979.

So what kind of information did SAE think technicians should be able to access with a scan tool? All of the following:

• Mode $01 - Data stream (sensor readings and switch status)

• Mode $02 - Freeze-frame data (if DTCs are present)

• Mode $03 - Diagnostic trouble codes

• Mode $04 - Clear codes and freeze-frame data

• Mode $05 - Oxygen sensor monitor

• Mode $06 - Non-continuous monitors (EVAP, catalyst, EGR, etc.)

• Mode $07 - Continuous monitors

•Mode $08 - Bidirectional communication (onboard tests) Mode 09 - Vehicle VIN, PCM calibration, etc.