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Sounds like a sensor issue on the source. The heating circuitry is on the main board, while it cant be ruled out, most commonly the source heater sensor assy is the part that is usually at fault. If you have another block to try on that source, I would do that first and see if all zones reach set point. The quad is coming close, but both zones will go off if there is a fault. Let us know what you find please .
This resolved it. Thank you James.
Come across another issue now but we should eb able to resolve it.