Functional circuit test (FCT)

Functional test exercises the product the way it will be used: power rails within limits, firmware boots, buttons and displays respond, sensors read correctly against references, actuators move, radios transmit and connect, calibration values are written. It is often done at board level after assembly and again on the finished product as an end-of-line test.

Good FCT is automated, fast enough to keep pace with the production line, logs results against each serial number, and is designed together with the product's test firmware.

Where it bites: slow, manual or subjective tests become bottlenecks and let defects through. Limits set too tight cause false failures; too loose and bad units ship.