I am working on a walk in freezer. 404a. Evap. Coil was frosted over. I defrosted the coil, both sides. Tested defrost timer and elect heat, working ok. Turned system on. Suction pressure was 25-30. Suction line temp was moving from 0 to 9 deg. Air entering evap was 24. Air leaving 18 deg.
As soon as I turned it on, the coil started frosting up. I let it run, but never wold drop below 20 deg.
This system was working and has been for a long time.