An improved pump, reservoir and reservoir piston are provided for controlled delivery of fluids. A motor is operably coupled to a drive member, such as a drive screw, which is adapted to advance a plunger slide in response to operation of the motor. The plunger slide is removably coupled to the piston. A method, system, and an article of manufacture for automatically detecting a force sensor failure in a medication infusion pump is provided. The electrical current to an infusion pump is measured. Based on the current measurements, the infusion pump detects when the plunger slide is seated in the reservoir, and detects a problem with the force sensor when the force sensor independently fails to register a value indicating that the plunger slide is seated in the reservoir.