loose belt or dirty sensor. remove the screw on top of the machine to allow you to swing the control panel out. Once you have the control panel out of the way you will be looking at the spindle there is a flat plate towards the right side which covers your sensor. there are two rings on the spindle one is for rpm one is for orient gap should be around .005. It is probably a most likely the belt is too loose If teh sensor is not picking it up there is usually a disconnect alarm.