Z level home shows Z+150

After auto home on my DIY 3D printer the LCD screen shows X 0 Y 0 Z +150 I can then move the Z axis down but only back up to where it was homed it will not any further up in the Z +.
Have checked all the limit switches done M119 all triggered. In the Marlin software the travel limits after homing are set at X max pos 200 ,X min pos 0 Y max pos 200. Y min pos 0.
Z max pos 150. Z min pos 0. I can move X and Y using pronterface in both directions and Z down but only back up the amount move down. Any suggestions to get home position to read X0 Y0 Z0.
regards Eric