No temperature is displayed.
If no temperature is displayed (–,- °C) and the sensor LED flashes white, there is an active Bluetooth connection between degree° and your smartphone but the monitored temperature is outside the displayed measuring range. See Troubleshooting > Display Ranges. If no temperature is displayed (–,- °C) and the sensor LED flashes blue, there is NO Bluetooth connection and no temperature values can be received. See Troubleshooting > Connection and transmission.
It shows a temperature, but I think it’s unlikely.
Check the correct fit of the sensor, the sensor head and its alignment of the sensor lens. The sensor head must point in direction of the ear drum. The sensor should lie on the auricle. See also > Applying degree° on the ear. Pay particular attention to the external influencing factors in the warning notices. These can influence the measurement results.