It could be either, however from looking at it the freeze pattern seems a little wierd which could be because of low freon. Because when freon is low you get a lower temp where the freon first enters the coil. So what can happen is it freezes just at the inlet of the coil. It looks like most of your coil is extremely clear, units not going into defrost tend to freeze more evenly.My outside unit has frozen up. Is this a sign of low refrigerant or defrost mode not working? It has rained here nontstop for 5 days. Outside temp is around 45 degrees.