using cat5 to wire sensors

I am not going to say the ONLY problem with using Cat5 or any other non-stranded type of wire is a reliability issue.  But that is at least one problem.  The wire as you know is a single conductor.  In an environment such as an automobile, it is subjected to a lot of vibration and flexing.  This is generally not a problem for the wire still inside of the jacket.  But once the jacket is stripped off of it the individual wires are allowed to move around a good bit.  The solid wire will break over time.  Just double up or triple up on the power and ground connections and use the remaining 2 conductors for the trigger wire. This way if 1 wire does break, the other or others will keep you working.

If you have it layin around....sure why not. But as idiot said, secure it well. If you need to purchase wire to do the job, don't spend it on CAT5.

Spend it on multistrand wire, the wire that should be used.