what is E75 on a vaillant eco pro tec boiler?

In the manual it says no pressure rise was detected on turning on the pump - water pressure sensor and or pump defective, air in heating system, quick bleeder defective, too little water in appliance, check adjustable by-pass, connect external expension vessel to return.
Ensure you have at least 1 bar pressure in system via guage or button, if not add some water via the linked water tap and pipe. Also bleed your radiators to ensure no air in system.

helpful
January 2010

I too have a Vaillant boiler, but E75 isn't in my Instruction book. However, if your problem was like mine, it could be that the outlet pipe draining the condensate away has frozen. The symptons sound similar in that I could turn the boiler on for a few minutes before it cut out again. On my Vaillant boiler the condensate pipe coming out of the boiler is a push fit. With a container held underneath the boiler, simply pull off this pipe. The water which has built up should flow out. This will allow the boiler to re-fire. I've cut the condensate pipe a few inches under the boiler and am collecting the water in a bucket. It comes in short bursts because of a siphonic action out of the container in the boiler, and should be a manageable amount which can be poured away each day. When the thaw comes, I will replace the output pipe aiming to get it better insulated particularly where it goes through the wall and outside. I hope this helps.

BH
January 2010