Perform the maintenance for both gas furnaces, replace the air filter for the basement unit, the homeowner had recently replaced the filter for the second floor unit. Performed a full measure quick scan on the system, found that both furnaces were running with a very high CO reading in the stack. Found that the manifold pressure for both gas furnaces were set for over 4 inches of water column, adjusted to 3.5 in as per the specification and confirm that the CO now dropped to the proper levels. Both systems are running and operating properly.