To enter diagnostic mode
1) Turn the key off and wait 10 seconds
2) Place the transmission in neutral
3) Hold the mode switch down and turn the key switch on. Release the switch as soon as the display is activated.
4) Use the mode switch (yellow button) to toggle through the diagnostic screens.
The clock screen is first,
then the battery voltage screen,
then the tachometer screen,
then the AWD diagnostic screen,
then the gear circuit diag. screen,
then the programmable service interval screen,
and finally the miles/kilometers toggle screen.
At least this is the process for 2009 machines, being an 800 yours is probably very similar.
Mike