Completed 4 system maintenance and inspection. I cleaned up furnaces and flame sensors. I checked all components and amp draws. I tested airflow and temp splits. Airflow pressure on both upstairs systems are failed. I tested carbon monoxide on 3rd floor furnace. After inspection heat exchanger I found where heat exchanger is leaking the carbon monoxide into the home. The exhaust is rusting through and the 3rd floor furnace does not have a designated gas shut off. Exhaust is improperly ran as well. Main floor systems are running within manufacture specifications but do have dust building up on the blower wheels. Talked with customer about systems and they would like some time to decide what to do.