Proper hotend heater for Reprap x400 Pro V3
The part causing the hot point failure was the connector for the heater and sensor. The connector showed no sign of burns on the heater pins, but the square tubes (see attached image) accommodating pins from the other connector had started to straighten out toward its original sheet metal state due to heating from current. Bending the tubes back in place allowed the connector to work temporarily and confirmed the failure cause. 

Details: After getting a 12 V 30 W heater, it also didn’t work. The hot point had stopped working because a wire brush had shorted across the heater terminals. Because a spare hot point worked, the failure wasn’t in the circuitry. The spare was a 12 V 30 W heater (from resistance measurements), while the heater on the failing hot point was a 12 V 40 W heater (from resistance measurements). This made me suspect that the heater resistance had shifted, and the circuitry shut off the heater. However, apparently both 12 V 30 W and 12 V 40 W heaters work on RepRap x400 V3. The brief short caused the bent sheet metal of the connector to start straightening back out of the bend.