Older roper side by side

I have a Older roper side by side i just bought to resale. I plugged it up and and checked that everything was working ( compressor , the fans and blowing cold ) and everything seemed to be ok. I put 4 bottles of water in the freezer side and I left it alone then today I checked on it and I noticed that the coils was trying to freeze up and the bottles of water was not all the way froze but it is blowing cold. I think it might be the defrost timer or the heater but I can't figure out how to put in in defrost mode to be sure of it. So if anyone knows how to put this fridge in diagnostic mode and what you think could be causing the problem pls let me know. Thanks