How can you tell if your heat exchanger is bad? I am getting high Co2 reading. And the flames seems to be blowing to strong, or could that be the gas valve, or dose it need adjustment in the tudes? I don't think the chinney is plugged.

If you are getting high readings, and your vent is not leaking or restricted, then that's a pretty sure sign that your heat exchanger is bad. How old is the furnace? Do you notice a change in the flame when the blower kicks on? Where are you getting the strongest reading from, near the furnace, or at the supply registers? Is there a gas water heater in the same room as the furnace? Any other gas appiances in the home, and if so, what are the readings near them? Lots of options here, but don't take any chances. Make sure to turn off all gas appliances and use electric heaters until the source is found.

All it needed was a good cleaning with a wire brush and a shop vac. Now the flames are nice and no co2 after retesting. Thank you for your reply. It was the first time it was cleaned in aleast 10 years.

Good deal. Glad things went smoothly for you. And thanks for the update, I always appreciate that.