Help needed identifying chip

Hi
appologies if posted in wrong area

I am trying to identify and locate a replacement chip on a Combi Boiler controller PCB.
I think it may be a Fairchild component but i have had no response from them.!

It is a 4 leg/pin chip with the following markings:

V921Y
f817A

the f is like a lower case italic which makes me think it may be the Fairchild logo??

The board is in a Remeha Avanta 28C ECO combi boiler ( and no, they have not replied either!!! )

Any help really appreciated.
Rgds
Kev

Got a picture?

OKAY Here is what I found out.
It is a Photo Transistor Optocoupler.
Fairchild Correct Part Number H11A817A

More info on the Boiler can be found here.
**broken link removed**