Aux heater problem

[e1ement]

I'm having problem with (water) Aux. Heater in 2017 T6 mutlivan. I'm unable to get the heater started using remote or overhead panel.
I get following faults in carista:

Infotainment display (roof)
Part #: 7E5919037G
Component: Dachdisplay
Software version: 0185
ASAM/ODX identifier: [none]
Coding: 000103 (hex)
Fault codes:
01299 Diagnostic Interface for Data Bus (J533)

Auxiliary heater
Part #: 7E0963272B
Component: STH-T5GP2
Software version: 0009
ASAM/ODX identifier: EV_AuxilParkiHeateWOST5G2
Coding: 5401 (hex)
Fault codes:
00373 Absorbers in the back right softly
00375 Control issuing window left open
00006 Manufacturer-specific code

Any ideas?

 

[mmi]

The auxiliary heater coding suggests that your van has a second battery under left seat.
The auxiliary heater fault codes (0373, 0375) suggest operation restricted due to insufficient voltage.
Have you checked the condition of the battery (under left seat)?