If there is a condition in which there is a gradual change in pressure (ex. gradually down to zero), the pump may stop working without throwing any errors. The pump error is setup to detect sudden changes in pressure (sudden spike due to blockage, or sudden drop due to a leak). This has happened when the inlet line ran out of mobile phase, drawing air instead of liquid.  Please ensure there is adequate mobile phase for running samples and that the inlet line is  placed at the bottom of the mobile phase bottle.