All new (or freshly-initialized) Apexes default to Fahrenheit. A temperature reading of ~20F means that (in order of likelihood):
- there is no probe connected to that temperature probe jack
- the probe is not plugged in and locked
- the probe is damaged or is defective
- there is a problem with the Apex or probe module
If you are not going to use a given probe port, do not just hide the tile on the dashboard - the proper way of dealing with an unused probe port is to simply disable it. For an Apex (2016), this can be done by using the Inputs page in APEX LOCAL or APEX Fusion. For Apex Classic models, this is done using the display module menus, or by using the Classic Dashboard (go to Configuration->Probe Input Setup). You can also rename the temp probe ports inn the same place.
Since you have a PM2, I am assuming that you also have a conductivity probe and will be using it. Since you have only one temperature probe, you will want to connect your temp probe to the PM2, and disable the temp probe port. Then enable temperature compensation on the conductivity probe port. Without temp compensation, the salinity/conductivity readings will vary with temperature - temp compensation reduces or eliminates that variance. PM2Salinity/conductivity temperature compensation requires a temp probe to be attached to the PM2. Start with an initial compensation value of 2.2.