Kenmore Ultra Wash Dishwasher - cancel/drain button is unreliable?

On my Kenmore Ultra Wash Dishwasher control panel, the cancel/drain button usually doesn't work. Every now & then when it's pushed it will work as it's supposed to. All of the other buttons that I use regularly work as intended.

Any ideas?

If this involves taking something apart, do you have a link to instructions for that?

Thanks a bunch for your help,
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