From my 2001 Service Manual:
(1) Vehicle speed is 0.
(2) Parking brake switch is pressed.
(3) Press the Display switch to display the Screen Adjustment screen. Display is next to the TRAF switch.
(4) Repeatedly touch the upper and lower bottom parts of the left end of the screen 3 times.
When Diagnosis System mode is started, system check is performed at first and the check result is displayed.
To finish: Turn the ignition switch from ACC to OFF to finish the mode.
Prius I - Diagnostic System Mode
