2014 Lennox single-stage furnace — no heat call. Found that the gas valve was not opening during the heating cycle. Performed diagnostic testing and confirmed the valve was receiving proper signal but failed to open. Determined the gas valve to be faulty. Installed a similar used gas valve from an older furnace temporarily to allow the system to operate and provide heat. Furnace is currently running and heating properly; however, this is not an OEM valve and is not a permanent repair. Advised client that a new OEM gas valve replacement is required as soon as possible. The current valve is for temporary use only and cannot be guaranteed for long-term reliability. Client agreed to go ahead and order the new replacement valve.