The fastest way to determine the problem would be checking the door interlock switch.
You should have continuity between BK and BU/BK with door closed.
Next, check if there is 120VAC at the door switch BU/BK and check continuity of this wire to the push botton switch.
If you have continuity and correct voltage going to the selector switch - replace it.
Find Appliance Parts at AppliancePartsPros.com - Largest inventory in the U.S. with 91% of all in-stock orders delivered within 2 business days. Thousands of photos and diagrams make it very easy to find the right part. Enter your appliance model here.