OBD II modes are organized into eight categories, each of which is assigned a particular mode number preceded by a $ sign to symbolize the hexadecimal code it represents. Full-featured, professional-level scan tools can access all eight operating modes, but inexpensive code readers or entry-level DIY scan tools can only access a few.
Mode $01 Identifies what powertrain information is available to the scan tool, lists the status of the OBD II self-check monitors (some of which are continuous or on-going while others are non-continuous and only run when certain operating conditions are met), calculated values (airflow, engine load, fuel trim, etc.), and all of the available sensor values (which are referred to as PIDs or parameter identifications). Some Mode $01 data may be available on some code readers and entry-level scan tools.
Mode $02 Is for more advanced diagnostics, and is usually only available on full-feature scan tools. This mode displays freeze-frame data (data captured and recorded when a fault occurs). How much data is stored depends on the capabilities of the vehicle computer. Some may store freeze-frame data for multiple codes while others can only store data for the most recent code.
Mode $03 Lists stored five-digit diagnostic trouble codes, and is accessible by all code readers and scan tools (though some code readers may only display P0 codes, and not P1 codes). Full-feature scan tools also can access other codes (body, ABS, HVAC, airbag, etc.) that are available on the vehicle.
Mode $04 Used to erase stored fault codes and to reset the OBD II readiness monitors. Available on most code readers and scan tools.
Mode $05 Available primarily on full-feature scan tools, this mode displays the oxygen sensor monitor screen and oxygen sensor test results, including minimum and maximum voltages. This mode is helpful for diagnosing O2 sensor problems as well as air/fuel mixture problems.
Mode $06 An advanced diagnostic mode available on most full-feature scan tools for reading actual test values for the noncontinuous monitors. Very useful for no-code diagnosis, detecting pending problems and verifying repairs. Test values for Mode $06 are displayed in hex code, which requires special software to translate or vehicle-specific reference charts.
Mode $07 Available on some code readers and most scan tools, shows the test values for continuously monitored systems. If the monitor has run and no faults are found, the vehicle can pass an OBD II plug-in emissions test.
Mode $08 Available only on full-featured scan tools, this mode allows bidirectional communication between the scan tool and vehicle so the scan tool can run various self-tests by sending commands to the powertrain control module (PCM).
Mode $09 Used to display vehicle information such as VIN, PCM