The zero code or “O” code means that there is a problem with the overspeed governor. The overspeed governor is on the back of the stairlift carriage and can be accessed by removing the covers. It sits in between the drive motor and the leveling motor. The purpose of the OSG is to stop the stairlift in the event that the stairlift is going to fast. The overspeed governor is like a mouse trap that needs to be set and can easily be tripped causing this error code. You can manually reset it by pressing a black tab at the back of the lift and making sure the plunger switch is held open by the white plastic tab on the plastic ring.