Vaillant vcw sine 18 not modulating.

30 Dec 2021
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United Kingdom
Happy new year all.

Ok, so I'm trying to fix my mums 40 year vcw sine 18 boiler which has given stirling service (20 years whilst residence was a guest house) and which I feel still has some years life left in it.
Having read several previous posts/answers from very experienced vcw heating engineers (hopefully still transmitting)regarding the possible faults and fixes for the boiler I'm now at this point ;
The boiler fires up when heating demanded (pressure diff valve pin operates gas valve)
The micro switch (new) at the gas solenoid operates
As temperature rises the flow/diverter valve rises and closes the left hand microswitch at the top of the valve
The boiler continues on full flame and temp continues to increase
Turning the thermostat has no effect at all and boiler continues on full flame until I turn it off (worried about overheating)
I've checked the ntc which seems ok, Ive checked the continuity of wiring from microswitches down to pcb and all ok, I've checked the edge connectors on the pcb and switch board and all ok.
I've removed the purple lead from the ntc and shorted it to frame and boiler does NOT cut out.
The hot water demand works ok but does get very hot.
Well......My humble opinion is that it must be a faulty pcb regulation fault and the board requires replacing. But before going further I wondered if any experienced voices out there could offer their opinion and whether I had eliminated all other possibilities.

Thanks in anticipation. Dave
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