Vaillant fault code F.76 is quite a worrying one. It means that main heat exchanger overheat protection had been activated. Vaillant Eco Tec range has a thermal fuse on top of the main heat exchanger that once activated breaks the circuit to prevent boiler from firing. That fuse is can not be reset and is not replaceable as once it is activated (at 140-160 degrees model dependable) main heat exchanger is deemed damaged beyond repair and must be replaced.

Before you go into panic mode though please be aware that this fault code may indicate a much less serious fault with wiring from the senor into the PCB. You will need a Gas Safe Registered heating engineer to confirm  what had caused your Vaillant to show fault code F.76. Thermal fuse blowing and damaged heat exchangers are rare and in most cases this code points to a wiring issue. Unfortunately on my last repair with F.76 fault code the boiler main heat exchanger developed a leak combined with blocked pressure sensor. Sensor did not communicate to the boiler PCB that there was no pressure in the system and kept the burner on although there was no water in the heat exchanger. This in turn activated thermal fuse to prevent a catastrophic meltdown. Picture below shows a partially melted thermal fuse connector.

This is how the main heat exchanger looked inside.

Client decided to replace this 8 years old Vaillant 637 with a new one. Since all connections, flue and brackets were exactly the same the job was relatively straightforward and the boiler got replaced, system flushed and commissioned the same day. It is worth to note that the faulty boiler had never been serviced. New boiler came with 7 years warranty provided it is serviced annually. New boiler installed below.