Vaillant vcw 242 eh and flame reset switch

Hi all first post just found the forum and need some help please
We have the vaillant vcw gb242 eh installed in house which will work fine but recently when c/h is on and I open a tap the flame reset switch will click out and will keep doing so until the boiler cools down and then it will work again.
Other times if c/h is on for a long period the flame reset switch will click out also.
The boiler must be 18 years old in that time it has had a new he unit, pump, and I have repaired one of the pcb when I could not get any c/h to work also I have just changed the monitoring electrode (Product Code: 090649) to see if that would help things.
We live in a soft water location no leeks on boiler (except he unit 15 years ago old black one) nothing seized diverter valve looks ok micro switch operating well.
Any help would be appreciated Thank you.

Quality boiler by the way,ive had an issue wiv the water valve sticking in hw mode situated at bottom of boiler.this will keep servo valve actuated for hw and may overheat.tbh faults with these cos there fairly complicated really needs an rgi there 2 see wots happening

combispecialist said:

Quality boiler by the way,ive had an issue wiv the water valve sticking in hw mode situated at bottom of boiler.this will keep servo valve actuated for hw and may overheat.tbh faults with these cos there fairly complicated really needs an rgi there 2 see wots happening

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great boiler unfortunatly also a complicated one. i'd agree with combi specialist here, you probs need an RGI to experience the fault and see what's happening. that lockout light can come on for a multitude of things

Thank You for the feed back anyone else