This works for me
- name: "Heizung Aussentemperaturfühler"
unique_id: "heatronic_aussen"
value_template: >
{%set outtempdec = int(value[1:],base=16,default=0) %}
{% if outtempdec < 0x7fff %}{{ outtempdec / 100 }}{% else %}{{ (outtempdec - 0xffff) / 100 }}{% endif %}
unit_of_measurement: "°C"
state_topic: "home/heizung/aussen"
state_class: "measurement"
