Methods, apparatus, systems and articles of manufacture are disclosed to diagnose a pneumatic actuator-regulating accessor. An example method disclosed herein includes stroking a fluid valve operatively coupled to a field instrument and a pneumatic actuator-regulating accessory by pressurizing an actuator operatively coupled to the fluid valve. The example method also includes blocking a supply to the pneumatic actuator-regulating accessory and the field instrument, exhausting pressurized air from the actuator via the field instrument, measuring at least one of (1) positions of the fluid valve or (2) an output pressure as the pressurized air is exhausted from the actuator, and identifying a first operation of the pneumatic actuator-regulating accessory based on at least one of a travel rate of the fluid valve or the output pressure, the travel rate of the fluid valve based on the measured positions of the fluid valve.