Well, we were just getting ready to
sell our Subaru legacy. It was running
fine, but my husband decided to put
a new battery in, just to be nice. Now,
there’s just a click sound and it won’t
start. Is there some sort of locking
sensor on it
No. Just charge the new battery, after assuring that the cable connectors are scrubbed clean. If battery won't hold a charge then you purchased a bad one.