This reports the BUC's internal temperature in degrees Celsius, returning -99 specifically when the reading is not available. Since the description notes this query is only valid when FSK comms and BUC power are both on, a persistent -99 reading does not necessarily mean overheating; it could just mean cdm570LBUCComms or cdm570LBUCDCPower is currently off, which an admin has to rule out before treating -99 as a sensor fault. A climbing reading from 35 up toward 70-plus degrees C on a BUC mounted in direct summer sun would warrant checking for a ventilation or fan issue before it triggers a thermal shutdown.