700 series ventilator low exygen error

Blease spaceblabs 700 Ventilator oxygen low, error, checked and replaced new oxygen sensor with old one, still error low oxygen, replaced sensor cable, then replaced PCB board where oxygen sensor cable is connect, at back of ventilator, still same error,, oxygen showing 0.0 and error say OXYGEN Low ...looking some expert tips to solve this problem (when cable is disconnected from oxygen sensor then on display show Oxygen "Err." if cable is connected it displays 0.0 and at top shows oxygen Low) ..if some one help plz