What would cause the display of the message "MPJ

This display message can be caused by some of the following:
Joystick out of neutral at start-up.
Bad or broken joystick gimble.
Bad or uncalibrated joystick gimble.
Bad wiring harness or connector from joystick
Internal joystick electronic component problem.
Bad communication chip in joystick.
Incorrect programming of the joystick
The most direct test is to run the joystick gimble in circles and forward, back, left, right, feeling for bad or dead spots and uneven action. Look at the position in neutral: left,right, forward ,back should be in alignment. A bad gimble is inexpensive and straightforward to repair.
Inspect the wiring for cuts, pinches and bad pins on the mating connectors.
With a hand held programmer (HHP) plugged into the main harness or charging port you can read the fault log. That way you can make a definitive diagnosis. You can also calibrate the joystick with a HHP and adjust the programming parameters.
If the chair powers up but the batteries are low you can get unusual electronic problems so test the voltage with a digital voltmeter. Voltage should be between 24 and 26 volts. It is possible but less likely you have a bad controller. Again, the HHP will read the register of all faults to help you determine the cause of your problems.
Check the obvious first and then swap out the joystick with a known good working one to determine definitively that the joystick is your problem. Hope this helps.