Upright freezer

I just purchased a used freezer and I am pretty sure something is wrong with it. It runs all the time, at first I thought that was natural cause we were putting fresh meat in it. But it has been weeks now and it is still running all the time. The temperature stays at 12 degrees. I tried turning it up higher and it actually heated it up so I have it on number 2 and as I said it stays at 12 degrees and runs constantly. What do you think is the problem?