Ariston microGENUS 23 MFFI C.H overheats & red L.E.D loc

[Nuttyboffin]

Hi Gas Boiler experts, please let me have your thoughts on this fault. With boiler C.H control knob set to "0" the boiler fires up as soon as it is powered up. The radiators heat up very slowly, (I cannot hear the gas valve switching the solenoid for high gas flow rate) Gas flame height looks lower (than what I normally see)

The water pump is running and radiators eventually get extremely hot and the red L.E.D lock out lights up and boiler shuts down. During this heating up phase none of the green led light strip (which indicates water temperature) light up. After 5 minutes or so, the boiler can be fired up again by hitting the reset button, with lockout fault re-occurring consistently a few minutes after.

When the hot water tap is run it's hot for about 10 seconds and then consistently luke warm (regardless of control knob setting.) Again flame height looks fixed and on low set point.

With so many things apparently not operating I was thinking to change the P.C.B. ? But before I do that, could it be the temperature sensor reporting an inaccurate low temperature and the boiler automatically firing up in a frost protection mode? This could explain it also eventually allowing the boiler to over-heat. But would the led sensor not light as the temperature increased? I guess these are also linked to the sensor response so probably not. Any body seen this issue before? Thanks.

 

[dcawkwell]

Temperature sensor. The boiler is overheating and locking out.
These are pants boilers to work on.

 

[Nuttyboffin]

By way of an update. I fitted a new "TEMPERATURE PROBE (C.H.W) ariston Pt. No. 569236. ( As suggested by dcawkwell) There are 2 of these sensors on the diverter valve body assy. but it happened to be the first one I changed which was the one directly below the diverter valve motor assy.
The LED array on the front panel of the boiler which indicate the temperature of the water in 10 degree increments are actually Yellow, not Green as per my original post.

So in conclusion it appears that the faulty sensor was reporting a temperature below 4 degrees centigrade and hence the bolier was automatically firing up in frost protection mode. The water temperature L.E.D's were not lighting up as the boiler heated up as they rely on the temperature sensor that was faulty. Hence boiler just kept on heating up until the overheat sensor kicked in and locked the boiler out with the red L.E.D lit up.

This sensor fault also causes the gas valve to run at the lowest set-point which means that the central heating takes ages to heat up.(before finally overheating and locking out.) It also causes the Domestic Hot Water to run luke warm as again the gas valve is on it's lowest set point, which appears to be programmed in the bolier software when in frost protection mode.

I hope the above is helpful to anyone else that suffers with Ariston boilers !! and big thank-you to dcawkwell and like minded people who support knowledge sharing on this forum.