Checked, they can gas fire furnace three years old diagnose W wire from thermostat was loose and not making proper connections removed W wire and rest, stripped and reconnected turn system on and checked operations and carbon monoxide. None detected left system working. View visual findings from this job here - https://visionshare.xoi.io/?id=XA-DE27-C5924060843046CC9DD7D19A52C18BAD