My spa won't heat even after replacing element

A faulty spa element blew the safety switch (GFCI?) so I replaced the entire heater unit on my “legacy” non-M7 Balboa spa, with a 25315 Rev 1 control board. The spa pumps and blower now work OK but the spa won’t heat. The new element, the pressure switch, the heater relay, and the high limit relay all test OK but there is no voltage getting to the element. I have also tested the two temperature sensors and they each read about 36 kohm at ambient temperature. Balboa’s diagram is only a very basic wiring diagram which is not very useful for troubleshooting. Does anyone have a more detailed schematic circuit diagram or some useful tips?