Worcester 14_19 fires up OK then turns off

[Dekkered]

Boiler gets Demand light,
fan starts and runs for 20secs and then the gas starts and ignition fires and flame shows and the Flame light comes on.
Almost immediately the flame dies, Flame light goes off, fan continues and the boiler then repeats the sequence with the same results.
There is no Lock Out light showing.

I replaced the Flame Sense and the fault has changed slightly.
The ignition fires and lights the gas but the ignition keeps firing as though the flame isn't being detected?? (new part fitted)
Also the Flame Light doesn't come on briefly but the result is the same in that it initiates another start sequence and the fan remains on.

This was an intermittent fault usually on start up first thing in the morning but after several tries it would catch and run fine all day turning on and off as demanded.

I'm thinking of PCB board but the change caused by fitting a new Flame sense has surprised me.

Any help appreciated.

 

[PrenticeBoyofDerry]

Could be a pressure switch issue, if the fan is shutting down.

 

[Dekkered]

Could be a pressure switch issue, if the fan is shutting down.

Thanks for your reply

The fan seems to be OK, it doesn't shut down.
I've checked the Air Pressure switch and it works OK