The Webasto heater has been a wonder of reliability for many years. But when we left Sweden it was hard to start. First I thought it was a fuel problem and changed the filter. Then I discovered it started if the voltage was around 14 V. Starting the engine every time to get the voltage above 14 V is unpractical in the long run.
I contacted Webasto who recommended to change the burner. They said, it most likely is carbon deposits in the burner.
I ordered the service kit from Martin at HR Parts. Burner and heat chamber comes in a kit with instructions how to change. New seals are also included. This work has been postponed for a long time. Not that we need the heater, but I needed a job!
You have to take the heater apart more or less completely to get access to the burner. 2-3 hours work from start to finnish.
Take pictures when you take the heater apart. They will be helpful when you put everything together. There are a number of cables that needs to be pulled out. Pay attention on the heat protection. You will see it when the mid plastic covers comes off. You want it back in the same place. It is not explained well in the drawings.
Heater started on the first try with voltage around 12V. Success!
Do not try just to clean the burner. It will only last a short time as the carbon that is absorbed in the material under the heat pin can not be cleaned out. I would not recommend starting this work whiteout having the service kit.
Update: Webasto sent me this picture today. The heat shield should be installed with half of it under the grey mid cover, and the other half under the black end cap. If there is a joint in the heat shield it should be located on top and in center were the heat sensor is located.