My AC (lennox) is not coming on when needed. The A/C is set to 75, it'll click but nothing comes on. I've checked the batteriies (changed them too), then I checked the fan. Turned it from 'auto' to 'on' - and nothing. There's been construction around here - redoing the streets, and I think the vibrations has knocked something loose. It's been looked at twice - and 'fixed' both times(both times they just tighted some wires next to the relay), but has happened again. Suggestions for a permanent fix?

If screws are coming loose, you can use Locktite 222 to secure them.

A friend came over and checked it out (they can fix anything!), we put locktite on the screws that were tightend, but found the fuse that hooks the thermostat to the ac was loose, he tightened that and replaced the fuse - all is okay. Thanks for the locktite idea - at least I know that won't happen again.

Yea, but you probably didn't use 222. It's a low strength.