According to the operating manual, the control panel should show faults that may help with diagnosis - https://dealers.webasto.com/service/...s_FileId=17842
Don't wanna sound obvious, but is the pump installed in the proper orientation? in vs. out?
Other than that, the manual isn't very descriptive for troubleshooting...
[from the manual above]
If the system is fitted with a combination timer, when an error occurs an error code is output on the timer's display until the heater has been rectified. The error code is output if the heater is fitted with a control element after an error has occurred by the switch-on indicator/error code indicator flashing. After a burst of fast flashes the error code will be output by a sequence of long flashes, the number of which is shown in the table below.
If serious malfunctions such as overheating or failure to start reoccur, the heater is locked (F 12) and can be put back into service by disconnecting the power supply with the heater switched on (e.g. by removing fuse).
Error code Meaning
F 00 Control unit error
F 01 No start (after 2 starting attempts)
F 02 Flame failure (at least > 3)
F 03 Undervoltage or overvoltage
F 04 Premature flame recognition
F 05 Flame monitor (petrol heater) interrupt or short circuit
F 06 Temperature sensor interrupt or temperature sensor short circuit
F 07 Metering pump interrupt or metering pump short circuit
F 08 Fan motor interrupt or fan motor short circuit or fan motor overload or blocked
F 09 Glow plug interrupt or glow plug short circuit
F 10 Overheating
F 11 Overheating sensor interrupt or overheating sensor short circuit
F 12 Heater unit lock-out
F 14 Overheating sensor installed incorrectly
F 15 Setpoint generator short circuit