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The Webasto Aux. Heater can be unlocked through VCDS. It may be come locked do to a broken water pump or simply failed starts.

The heater is locked if it reports the following fault code:

205935
00 0018 03 1232 thermo top v 12v /42 w 5kw

Saturday,05,November,2011,16:56:54:21701
VCDS Version: Release 10.6.5
                Address 18: Aux. Heat       Labels: None
Control Module Part Number: 1K0 815 071 R
  Component and/or Version: Zuheizer        050 2302
           Software Coding: 0000011
            Work Shop Code: WSC 01324 785 00200
                      VCID: 346C9F582EA0
1 Faults Found:

02251 - Heater Unit Locked
            000 -  -
             Freeze Frame:
                    Fault Status: 01100000
                    Fault Priority: 2
                    Fault Frequency: 1
                    Reset counter: 145
                    Mileage: 204126 km
                    Time Indication: 0

The heater can be unlocked through VCDS by:

1. Selecting the “Select Control Module” option and then
2. Select the Aux. Heater:
   
3. And then select the Adaptation:
   
4. The save 1 in channel 42.
1. That’s it.
2. Clear the fault codes and read them again and you should not get the heater locked fault any more.