I have screw type genie pro88 model.It does not have sensors mounted at each side of the door.The door only open half then the door stopped (I have to hold the opener's bottom all the time).If I press the bottom again, the door go down couple inches then go forward couple inches. If I press it again, it still go back and forth automatically.Ends up the door is still at the halfway open position. I check the carriage. It is still in good condition. No worn out sign.
Is it possible the control p.c. board relay goes bad?
Any body can help me? Thank you very much!

Pull the disconnect and see how easily you can open/close the door by hand. Ut should not take more than 25 lbs of pressure either way, could be your springs have slipped or are out of adjustment.