Webasto Troubles

I have a Webasto engine heater causing us some grief.

It will start up and run for approx 3 minutes and then shutdown.

Here is what its not:
fuel related (filters, lines etc)
exhaust
glowplug
fan
fuel pump
the old pull the fuse and reset it trick

I've been told perhaps the waterpump isn't circulating but wouldn't that give me an error code on my timer?
I suspect a air lock because I think the heater thinks its warmed up the water enough and then it shuts off but doesn't give a code because it doesnt sense a problem. How do I bleed these? (pretty sure its a 50).

Is there anything else I should be checking? Low voltage maybe? But if its that issue then why would it fire and run at all? Could the timer be the problem? It keeps going as if it is still running so I'm thinking its not the problem.

 

Its a Thermo Top 50 with the digital timer 12 volt. Not too sure why the truck would make a diff since it isn't wired to anything but the battery.

 

OK thanks guys. It turned out to a be a very weird air lock. We ended up changing the water pump over it and it still didnt fix it until we actually pressurized the lines and forced it to cycle. Even with being bled it still had a bit of an airlock. We were not getting a code because the heater thought it was heating the water up to temp.