So I made a "vehicle" called sauna. The problem is that mostly every time when I enter the sauna the character leaves from it. Sometimes it stays in the sauna but mostly it exits. What have I done wrong?
It happened to me once when I had forgotten to add a NetworkableInfo to my PCO. Make sure that sauna_heal_body is defined.
The fact that your PCO has no LodObject (or any kind of geometry) may also be a problem. Theoretically it's not needed, but BF doesn't like it when you don't add it.
Also, try adding hasMobilePhysics 0 to your PCO.
Anyway, if I were you, I'd rather copy flak38 or aa_allies and then modify it to do what you want it to do. In BF there are lots of properties that in theory are not required, but in practice the game goes crazy when they are not present. That's why it's always safer to copy existing vehicles and work from there. I always do my static PCOs like that.
If you happen to have the Galactic Conquest mod installed, you can take a look at bacta tank, it seems to do exactly what you want.
Also note that network info is case sensitive. If you define network info, the network info template you create must also match what you used on the PCO letter by letter. That includes any capital letters you may have used.
Simply looking at threads in the past would have answered the question for you.
We have many threads here on this forum, and so most of your issues have been found by other people.
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The instantexit always is caused by messed up networkinfo. Heed the info the others elaborated on and you should be fine. NetworkInfo is not very obvious to understand, but it's relatively easy to go with a I-don't-know-what-I-actually-code-but-it-works-so-who-cares approach. Just take a look at the network.con of any other object and code it according to the same template (always use Linear method, that is the correct one in 99% of cases and the most stable one anyway).
If you'd try your map on a LAN server (dedicated one, even local) you would get an insta-CTD due to the faulty networkinfo.