2 system furnace tuneup and signed for a new Comfort Club Membership. Replaced very dirty filter, checked electrical and safety switches all ok at this time. Both systems have rusty evaporator coils. Both furnaces have worn ignition, weak flame safety sensor, and weak blower motor controller. Did a combustion test on both furnaces. Upstairs all within range. Found downstairs with higher oxygen level. Recommend a combustion calibration to make furnace burn cleaner and more efficiently.