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It is possible that refrigerant is low and not low enough to detect a error code.
Cold Outdoor temp has a big impact on performance even on a perfect installation.
The exact amount of refrigerant is very important and cannot just be added without refrigerant tool such as scale, gauges, adapters and knowhow....
we first have to determine if there is a leak in system and fix the issue if there is, it is not normal to loose refrigerant in any way, even after 20 years....,
This issue should be performed by a qualified technician ONLY.
Just adding refrigerant even if you were able may damage unit severely..
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