In message .com,
" writes
The Potternton EP2002 is a timer control unit for a boiler, mine being
a Prima F.
I came home to discover I could not manually press ADV, which would
normally activate the HW or CH, this is confirmed by the red colout LED
on the right hand side illuminating.

Nothing illuminates at all, but the the time is displayed and is
correct.

I also discovered that I could not set the time, or modify any other
settings on the timer unit.

I removed power to the unit, (fuse isolation) and waited until the LED
displaying the time faded. But when I turned it back on - SAME
symptoms, I also noticed the time was exactly the same. So either this
unit has a large capactitor in there, or a back up battery keeping the
time backed up... I don't know.

I do know that I cannot control my heating.

Yes, it has a battery for back up

It's so long since I repaired one I can't remember, but IIRC there's a
battery in the back

unscrew it (2 screws on the bottom face and rotate it upwards to remove
it from the backplate



--
geoff