Performed preventative maintenance for a carrier furnace model #58sta090-16, serial #1918A16292. Upon arrival the system was off so I turned off the power and removed the access panels. I removed the igniter and flame sensor. Cleaned out the burners and burner box. The flame sensor is in very rough shape. Checked all electrical components to ensure that they are secure and in good condition. I did find some thermostat low voltage wire that were tied together but not capped off. I used wire nuts to cap off the low voltage wires. The homeowner replaces their own air filter size 16x25x4. I reinstalled the access panel and powered the system back up. I called for heat and took a combustion analysis test. I received the following results, o2 is at 8.5% and the co levels are 16ppm. The homeowner requested that I check the incoming gas fittings for leaks. I performed a bubble test and found no leaks. Performed preventative maintenance for a carrier ac system serial #2318E16343 model #24abc6