I have a treadmill york barbell max series 2, keeps displaying err in the speed window and wont start up at all, sometimes it will start working and then sometimes it wont and err comes up again

1) You have out of adjustment the sensor reed switch,see the schematic.by 1/8" or 3mms aprox.
2) With this you will fix it.
3) However if the problem persist, you must check if the display board is o.k or faulty.
4) Test the 5VDC for IC( Including EPROM, ROM,etc) Tight flexible cable for Display Board vs Lower Board(Main Controller Board).
5) Since the Lower Board send the Power Supply coming from the Transforme you must Test it, and then power supply for +5Vdc.
6) You must have Strong Conclusion if the problem is Upper Board or Lower Board(Main Controller Board) .
Finally The Lower Board Control 90% functions of the Treadmills.