Glow worm 18hxi overheating high limit cuts in but display reads 35o

[swindon]

display says 35oC but goes to high limit cut out
cuts out at 87oC
changed flow/return thermistors but still the same
differential on hot water 9oC
is it a pcb fault

 

[bewsh]

Could be a fault with the pcb misreading the thermistors and showing the wrong temperature.

But, is that causing it to overheat? Is there circulation issues? Differential across plate hex seems low, is the water going around the bypass?

 

[swindon]

talked to glow worm whilst testing ohms resistance and diagnosed during visit
it was the main pcb
took both pcbs as was not sure b4