Is this plumber lying about the faults? (Boiler repair)

A boiler is just like a car. It needs a service and an MOT.
We have experienced this before, how olds the boiler? and do you have the manual or is the heating engineer working off the manufactures fault finding process? sounds like the engineer is being genuine. Sometimes from a manual a fault can be shown and then come across another fault. Sometimes if the gas valve is playing up or not working the fan will not work causing the boiler to lock out as a safety feature.

He is just changing obvious parts until he gets it right, if he knew what he was doing he could have tested components with a multi meter. Boilers go through a sequence before they light, or give a fault code to give you an idea what to check

Each part of a boiler can be independently check and repair or replace by a competant person ( gas safe registered engineer).

The gas valve could have been tested without replacing the electrodes. Don't think there's a con going on though as it could have been a simple mistake if the electrodes were also faulty and the plumber presumed that was the only fault.

He is correct and some faults are multiple and you have to correct them one buy one