Potterton Performa 24

[spike.gb]

I’m having problems with a Potterton Performa 24, it works fine on hot water. When we call for heating it flashes up for 2 seconds then goes to reset with the case off. When the case is on it lasts a little longer but doesn’t recognise its alight. Tried a new board to no avail still the same. Electrodes and leads are good, thermistor resistance ok, pump proving switch makes ok. Totally puzzled how it can work fine on hot water yet keep sparking on heating until it goes to reset. Help please.

 

[Gasmk1]

Could it be gas valve as for hot water it would open gas valve fully but on start up for heating it would open on low first before modulating to full. Just a thought

 

[Last Plumber]

Burner pressure ok?
1.8 mbar min

 

[Frog67]

Divertor functioning ok?