A fluid delivery apparatus for delivering fluid according to a fluid delivery rate command including a fluid tank, a pump, a buffer, in which a pressure sensor is positioned, an injector, a pump controller, a fluid delivery controller, and a diagnostic controller. When a motor driven pump is used, the diagnostic controller is able to detect issues by comparing a measured pressure change with an expected value calculated using pressure sensing values and the power applied to the motor. If an air driven pump is employed, pressure sensing values, fluid delivery rate commands, and pump operating status can be used for calculating compressed air volume in the pump and trapped air volume in the buffer, and for further triggering pressing and suction strokes of the air driven pump, diagnosing issues in the fluid delivery apparatus, detecting fluid level in the fluid tank, and refilling trapped air in the buffer.