Start at the control: read the current fault/status code from the board LED or service display, then step through the ESA diagnostic routine in the service manual, which reports input states (activation sensors, safety sensors, locks) and lets you command test cycles. Confirm each activation and safety input toggles when triggered, check supply voltage, and watch the motor current/behaviour during a manual cycle. Documenting the displayed code plus which inputs read active narrows the fault to sensor, wiring, motor/MDU, or control board before any parts are replaced.