The way it is typically done outside of a maintenance mode is to turn the printer off, then turn it on. During the power up sequence the head moves to the left away from the parked position. When it does that you pull the power plug (not power down the printed with the button). If you have a battery backup UPS make sure you pull the plug from the UPS, not the UPS from the wall (don't ask me how I know).
Not sure if the P900 moves the same way as other wide format printers but give that a try.