<!-- from fixya rte -->Some buttons on microwave keypad won't work - KitchenAid KCMS1555RSS Stainless Steel Microwave Oven

press start button twice it sounds as if it is still in child lock mode

From your description you have a bad door switch, there are usually a few of them located inside the microwave under the keypad control board that will need a digital meter to trouble shoot.

The keypad control board is defective and must be replaced.

The keypad control board is the problem and the only solution is to replace, however! part will exceed cost of some microwave ovens.

It is most likely the keypad control board has been damaged, first! unplug the microwave oven for 5 minutes, if this works the micopressor will reset, if it fails the control board will need to be replaced, a costly venture.

The problem is in the keypad control board, or smart board. I recommend a estimate because cost will be high.

Try:
1) Unplugging power and plugging back in
or
2) Press Cancel twice while door open (works on my Whirlpool).

Very angry. Fairly new, little-used Spacesaver Microwave [3][Clock][Surface Light] buttons now do not work. We never even press [3], just [1] and [2] to warm sandwiches.
Unplugged for an hour. Still not working. Everything else works. We never use anything else. Bad solder joint in circuit board or keypad?
Thank you.

Membrane keypads are not very durable, and do fail after a relatively short time.

The only method is to replace them as a spare part.