Baxi 50/4 back boiler fault.

Boiler runs for @ 30 mins then ticks for half min then cut-out light comes on! engineer has replaced the 'spark gap' h t leads, & yesterday the pcb, he
considered the gas valve but found during 'ticking the was no volts on the left side of the gas valve! he's at wits end any suggestions appreciated! Roy [we can 'reboot' it by turning on/off, then it works for another 20 mins!]
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