Oil fired boiler

[Edufixer]

Grant boiler will not fire when override switch is pressed on programmable unit.
I have replaced programmable unit but still not working. It is working within timings and the hot water override works on demand.
Any ideas what to check next?
Thanks

 

[oilhead]

What 'programmable unit' do you have?

 

[Edufixer]

Honeywell st9400

 

[oilhead]

I would be looking at other components on the controls, such as 3 port valve. If it is a recently developed fault, you may be best served by consulting a professional.
The fault finding sequence for y plans etc are available on the Honeywell web site, and if you are able to use a multimeter, are easy to follow.