Diagnosis
A faulty membrane switch on the control panel. Spills or repeated use can cause the conductive pad under the button to wear out or become unresponsive.
How to Fix It
First, clean the keypad surface. Unplug the microwave. Dampen a cloth with isopropyl alcohol and gently wipe the entire control panel, pressing each button several times. Allow it to dry completely. The alcohol can clean away mild conductive residue. Plug it back in and test.
If That Didn't Work
If cleaning fails, the membrane switch assembly is likely damaged. On this model, the control panel is a single sealed unit. It cannot be repaired; the entire panel must be replaced. You can use alternative buttons: for '2 minutes', press 1-0-0 for 1:00, then +30 Sec twice.
Heads Up
Do not use excessive water or cleaning fluids, as they can seep behind the panel and cause further damage. Avoid using sharp objects to press the button. Continued forceful pressing will worsen the damage.