Check carrier gas fired furnace 18 years old diagnose system is not turning on due to one fan in the can is not sending 24 volts From the air proving switch also further diagnostics proves the gas valve is not opening even after 24 Volts is Is applied after jumping out 24 V from the air proving switch located in the Fresh air intake also known as fan in a can recommendations are replacement of fan in a can and replacement of the gas valves Due to age of system, highly recommend a replacement Please see photos for details office personnel please follow up with the homeowner for a quote on gas valve and fan in a can replacement View visual findings from this job here - https://visionshare.xoi.io/?id=XA-A243-AA0F450F106A4B4CBA3E25449FE70715 left power supply and gas valve off as safety precautions