Auxiliary heater not working always

[emog79]

T6.1 2022 california ocean. Full of diesel, full of battery . Sometimes the heater wont start, only on the light simbol on panel and after 1 second or 2 the simbol go off. The way i found to start heater is with ignition on or doing on-off ignition. Seems that on off ignition resetting some kind of fault in starting.heater. what do you think about? Is possibile to see what fault heater is getting?

 

[emog79]

The yellow light goes on and after off and the heater not start. The only way i found is startj g the heater with ignition on (no engine). Why?