Complete 2 system furnace repair and preventative maintenance check. Replaced dirty filters, checked electrical and safety switches all ok at this time. Replaced bad flame sensor for downstair system and Upstair flames sensor is weak. The owner wants to wait for the upstair flame sensor. Replaced Downstair flame sensor and downstair is heating normal. Did a combustion test and furnace is safe to operate at this time. Both furnaces heating normal