I have a tempstar NTC6 125KJGI. I turned it off at the kill switch attached to furnace the other day to clean the filter. After cleaning the filter it now blows cold air. However, I think I see the gas igniting when it kicks on. Blew hot air before. 2 months ago an inspector said it was in good condition. It is about 5 years old.
Thanks for any help

Kill the power again. Remove the blower door, check to see if you removed a wire from the blower limit switch (small device with two wires going to it). Should be mounted on the side of the blower housing. You may have hit it when taking the filter out.