Schreiber APM6855 Defrost Problem

I fitted a new defrost sensor to this intergrated F/F after testing heater and thermal cut out and finding them to be ok. I also checked the small board at rear behind condensor which was fine. I got called back after 2 weeks to find it frosted up again. There is only the main board left. I did 2 more of these last week. Same thing all defrost system tested ok but evaporator frosted up. Replaced sensor even though the old one had the same reading as the new one at room tempreture. Waiting for the calls. Whats the cure for these F/F’s.

Thanks

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