Baxi 105e problem - CH fine, but not hot water

Hi there - bit of a long post, so apologies.

We've got a Baxi 105e in our flat, working fine for 2 years up until a few weeks ago when it stopped heating the hot water above 40degrees, but CH still working fine. No flashing lights, boiler appears to ignite but the temperature only goes up 1 or 2 lights.

Called a plumber who initially thought it was diverter valve(?), replaced it, but it didn't work. His next idea was to replace the PCB. Because we're moving flat soon he suggested that instead of buying a new PCB he instead reduce the max flow so that we still get hot water, but at a low flowrate.

Anyway, after a couple of weeks this slow flowrate is driving us potty (the initial plumber is not returning calls). So I wanted to ask 1) is it likely to be the PCB on the symptoms I'm suggesting 2) can I, as a novice, replace the PCB and 3) assuming the first two are OK, is there any problem increasing the flowrate again and if so, how?

Sorry to put in a long message, done some searching but can't find exactly what I need. Thanks in advance, K.

check for diverter letting by, turn off heating, allow to cool, run hot tap for a few minutes and check for heat on flow pipe.

More likely to be the plate heat exchanger is blocked or the dhw thermistor is u/s

Thanks for the quick replies.

If it was the heat exchanger being blocked what would cause this. Live in Scotland - so unlikely to be limescale. Have done some radiator removal in the last couple of years, so could it be corrosion problem?

If it's not a daft question, what is u/s?! Is this the problem that the control dial is offset, or disconnected from whatever is inside (or have I got the wrong end of the stick, or the wrong stick entirely?)

Ta, K.

U/S broken paggered gone to the thermistor house in the sky

mind if you want to be nasty united states

u/s = unservicable