Here is a (certainly not exhaustive) list of error codes for the Eberspacher Airtronic D4 air heater. If you know of others please let us know so we can add them to the list for the benefit of others.

011 – Low Voltage

031, 033 – Blower motor not running

051 – Check fuel supply (this can indicate a whole range of problems including no fuel or the unit being sooted up…)