The three selector dials on the control board are actually encoders that send electrical pulses to the computer to tell it how far you're turning the dial and which way. All 3 encoders on the board are the same. They go bad after a while. The Temperature/Darkness encoder on my oven does not work at all. I did some testing on it with a multi-meter vs the other two which are working and I could tell from my tests that my Temp/Darkness encoder does not work the same as the 2 working ones so it needs to be replaced.
Look at this thread for instructions on how to get the control board out of the oven.
Fixing the start button
If you scroll down far enough in the discussion, you will see that someone offered a better way to take it apart so that a series of 5 black screws aren't so difficult and remove.
As far as I can tell, the encoder used is a Bourns PER231-P19-N0010 low profile 23 mm 10 pulse/20 detents per revolution mechanical rotary encoder available from both Mouser and Digikey. At the time of writing this, only Mouser has stock on these so I will be ordering from them. Cost is just over $5 CAD so probably ~$4 USD each.
If you want the US and not the Canadian Mouser and Digikey links I think all you have to do is replace the .ca in the links with .com.
I recommend you replace all of the encoders at the same time because you never know when the other ones are going to fail in the same way. Replacing them on the control board is pretty straightforward and easy if you know anything about soldering and removing and replacing components on PCBs. There are only 5 legs on each encoder and they're easy to get at with a soldering iron without touching anything else.
I will report back with the results after I have received my parts from Mouser and have installed them. I will be replacing all four micro-switches on the board at the same time. Read the Fixing the Start Button thread I provided a link to for information on what switches to order as replacements for those.