Diagnosis
The door hinges are worn or the latch mechanism is misaligned. This prevents proper engagement of the door interlock switches.
How to Fix It
Inspect the door hinges for looseness. Tighten any screws on the hinge assembly. Check the latch hook for damage. Adjust the door position so it closes firmly against the switches.
If That Didn't Work
Replace the door interlock switches if they are worn out and not making contact consistently.
Heads Up
A misaligned door can cause microwave leakage. Have it checked with a leakage detector if unsure.