best way to find out is to remove the user panel by a small screw underneath and fire boiler.the heating should run at max if it does replace user panel because has faulty control.had this a few times due to people bleeding pump over the user pcb
We are sitting here in suspense Paul.
It sounds like a stuck diverter valve so the boiler thinks the ch is up to temp.
I have had one before where the user control was goosed (this has the potentiometers in it)
Very rare though i had went down the diverter valve head and body route before finding that one.
i took the cover off the front and had a look.
thie first thing i saw was a sticker saying thay it was a HE30 not HE24.
i think that the diverter valve is stuck as the water to the valve is hot and the water after the valve is cold. the valve clicks when it tries to work, im going to take the solenoid off saturday to see if thats it.
not sure why some think its anything to do with gas valve etc , dhw ok , flashing c on ch call but not firing and not going to lock out is always divertor on isar, boiler thinks its up to temp due to being in dhw mode on divertor