Customer thought burning smell came from vent from addition system furnace. I checked furnace. Checked new circuitboard and found no issues. Checked blower and found capacitor getting weak: 17% low. I replaced capacitor. Burners cycling. No fault codes. Tested flame sensor current: 2.8 micro amps. Cleaned left pressure port. Vent that customer smelled burning smell from was actually part of the main system. I checked main system for any problems. OK. Suggestions: Heat from the addition system can incidentally warm the thermostat for the main system, causing uneven temperatures in the house. Suggest adjusting thermostats accordingly for this issue.