This OAT probe is required to read outside air temperature (OAT). Though Aero 553 works fine without this, the OAT temperature display will show *** for outside temperature. we use a standard OAT probe found in General Aviation 0-5 Volt. If you have an existing probe, you should be able to use it but cannot be shared as this is a serial resistance type ciruit so each OAT guage must have it's own probe.