Just had same with mine (ideal c80ff).I tested everything then opened up the ignition PCB which lives on top of the gas valve , was all burnt around one of the resistors , got a new pcb and now working fine.I have had this as a problem for a few years now ie: DHW causing system to lock out, and having to put CH on to help me run the bath etc.
Consulted a electrical guy I know, and showed him the old PCB , he said that it had been a dry join that was arcing and that if would cause a intermmitent fault, which would over time get worse till it failed , which it did just befor Xmas .
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