Checked gas fired furnace diagnose high limit switch was tripped due to Dirty filter, replaced filter supplied by occupant off the house also due to the Blower wheel being impacted with dirt system is over 20 years old highly recommend a replacement due to age of system cleaning the blower wheel is an option. System is up and working at this time checked air temperatures and For carbon monoxide at the supply air grills, no carbon monoxide detected left system working note because of the age of the system after the cleaning off the blower wheel it may be unbalanced option for replacement of the blower wheel is also a option View visual findings from this job here - https://visionshare.xoi.io/?id=XA-A243-277B53BD87DC43CE8FA577F4281242A7