Dryer completes cycles normally, but occasionally "refuses to turn on." Pressing "control on" produces no lights or sounds (except it will lock controls as designed after holding for several seconds). Power supply is fine, and multiple diagnostic tests produce same result- everything works properly every time (buttons, LEDs, cycle selector, heat, drum spin, etc.) with no error codes.
I cleaned all contacts in the machine control electronics assembly, then performed the button function tests with multimeter on the P3 and P4 ribbon cable contacts per factory tech sheet- all buttons test OK. Haven't done same for rotary cycle selector yet, but all indications seem to point to bad machine control board, per tech sheet....
After sitting for a while without power, machine often sometimes function totally normally for a few more loads, then do the above again when we attempt to start the next load. Is there anything else to try before ordering a new board?
(We are original owners of this Duet set- power supply is modern and has not been altered since machines were delivered)

Nope- no lights whatsoever when it refuses to start. The "control on" button simply does nothing during these events, even though it tests "normal" during self-test and with multimeter.

You were right about dimly lit LEDs! I went back and powered it up in the dark, and 2 dial LEDs were VERY dimly lit. I did another test cycle and the dial didn't work right for the first time- it went crazy with lights and etc- I now think the interface panel is haywire (and, as you suggested, the control panel may not be bad). I'll read through what you posted- thanks!