A Vaillant F78 fault is very unusual and not listed in most manuals.

Searching online will tell you it has to do with the hot water sensor and its communication with the PCB.

But if you talk to Vaillant, (which you should because they are great) they will tell you otherwise.

An F78 fault is only valid on the mk1 range of Ecotec plus and pro boilers.

It indicates a problem with the readings from temperature sensors on the flow and return pipework inside the boiler.

Vaillant recommend :

• Checking for signs of water on the sensors inside the boiler case.
• Checking for water on the harness connecting to the flow and return sensors.
• Checking for water on the pcb where the harness connects. Make sure you are checking the correct connections.

You must be gas safe registered to open the boiler case. Please contact a qualified engineer or Vaillant.