Duet Sport Dryer, PF code, now no power

I have a Whirlpool Duet Sport dryer that displayed the PF code for about a week. Simply turning dryer off then restarting it seemed to fix the problem. Then one day it didn't power on at all anymore. I read from other forums to try to replace the main control board (which we did) and the thermistor (which we did) but still no power. Checking power with a voltmeter, we have 240 V at the outlet and there is power running all the way to the relay panel. But a red wire from the relay power to the main control board shows no power. (The black wire shows power.) There are no ribbon cable connections so nothing seems loose. What now?
Thanks!