Heat performance tuneup x 1 system. Changed out client supplied filter. Inspect blower, evaporator and ductwork. Test all safety features high limits and rollout. I had to disable second stage on the furnace due to 7 to 8 ppm carbon oxide. Advised Emilie that it’s the beginning of the end for her furnace. System is 20 years old. I advised Emilie that systems that old are very unpredictable and can be very expensive to repair.