Pentair Minimax NT TSI w/ DDTC won't start

My Minimax NT TSI w/DDTC does not start. The green power button is lit up on the DDTC but the display only shows "OFF". No error code is shown, but the heater never starts. I replaced the thermister probe (part number 471566) and the water temperature sensor (P/N 520272), which was reading 10°F low. Neither of these items fixed the problem.

If I turn off completely, the screen reads 888. When I turn it back on, the Power LED illuminates and the screen reads "OFF". No matter if I use Pool Mode, Spa Mode, or Remote Mode, the heater never starts. Any recommendations about what to troubleshoot next? Is it possibly the gas valve? The blower never starts How to test this?