With Hot Water and Central Heating OFF, the boiler fires up as soon as the switch is moved from Standby to On. So it burns for 30 seconds, the water overheats, and the safety cutout shuts it all down. Is it the pcb or a sensor failure ?
"Standby to On" is the top rocker switch, and this fires up the boiler, even with the HW turned off and CH switch set to off.
The red LED on the pcb is on
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