Fault codes covered
51-43
The outdoor air thermistor determines operating range of compressor and fan speeds in cooling and heating operating modes in conjunction with the OCT sensor. So if the OAT sensor is open or shorted AND the OCT thermistor is also open or shorted, the system cannot safely run and will shut down. Once the OAT and OCT thermistors are fixed the system will operate normally.
Source: ACHPVSR454-02SM troubleshooting pages.