Vaillant ecoTEC plus 618. Fires then dies

[Timmyturtle]

As per title,my boiler goes throught the ignition sequence,fan comes on,burner ignites then 2-3 seconds later burner goes off and boiler goes into pump over run.
Been fine for 4 yrs no problems. System on fitment was all new,with Vaillant unvented cyl

Any ideas please?

Thankyou

 

[Timmyturtle]

I have turned up the dhw thermostat on cyl ,and the boiler runs on hot water ok

However,the CH 2 port valve does send a signal to boiler to come on(then dies out almost straight away as above)

 

[lightning]

If hot water ok its not the boiler, look at 2 port ch valve