The most common thing is the least easy to diagnose! The door catch has a complicated mechanism that breaks rather readily. Even if you take it apart ( which isnt easy ) you'll be lucky to spot a working one from a failed one. And even luckier to not break it for sure.
There are three terminals: one goes to live, one goes to neutral and the other should becomes live shortly after first two are connected to their respective points... the failure mode is when it doesnt. Not easy to test though, as it's screwed to the front of the machine and all the Aquarius' I've seen need you to take the whole front off to test it in situ.
Usually diagnosis is made by replacing it and noticing that it works now! It's about a tenner. It can be replaced from the top if you remove the two screws from around the door catch. Fiddly getting a new one in that way... but possible.
They are adjustable ( little screw accessible on the back ).... but I've never risked that.
Sometimes when they're starting to fail, you can "tap" the machine near the door ( maybe slam the door gently and with great care! ) and it will start, maybe even run OK for a few days! But it tends to get worse as time goes on.
J Skaife, April 2005