Tor, it's parameter #47 under the Autostart Menu:

To enter the 4:AUTOSTART menu, first press the SET key and then use the Up/Down keys to switch between the available parameters.... 47: EGT/AMBIENT

Use this parameter to adapt the temperature sensor (EGT zero point) to the current ambient temperature
Standard value: e.g. 21 °C (70 °F)
After a re-installation, this setting should be checked and corrected if necessary!

To change a setting, first press the SET key and then use the Up/Down keys to switch between the available values. A ? indicates the value to be set. To finish the setting and store the new value, press the SET key again.