Your boiler's computer parts are not communicating correctly because they haven't been told which specific model of boiler they are installed in.

When replacing both components [display and electronics], after being turned on, the appliance goes to fault and displays the error message “F70”.

What causes this fault?

This fault occurs when the boiler's internal computer boards lose their identity and no longer know which specific model they are controlling. It most commonly happens after a repair where new parts have been fitted but haven't been digitally linked to the boiler's unique settings. Without this specific configuration code, the system shuts down as a safety precaution because it doesn't know its own power rating or technical limits.