I had a plumber come round in attempt to fix my boiler. However he charged me for the electrodes which didn't fix the problem and claims the gas valve also needed replacing, but needed to connect the electrodes to actually know that fault.
Is this actually true then or is he conning me for my money?
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.
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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.
Each part of a boiler can be independently check and repair or replace by a competant person ( gas safe registered engineer).
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