This OAT probe is required to read outside air temperature (OAT). Though Aero 553-500 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 already have a probe, you should be able to use it, but the two cannot be shared because this is a serial resistance type circuit. Each OAT gauge must have its own probe.