Performed preventative maintenance for standard gas furnace. Customer also called for system not working. Upon inspection I found the inducer port to be partially blocked causing intermittent operation. Cleared the debris and cleaned the system. Heater ran as it should after port was clear. Recommended the customer call a chimney company to inspect the flue going to the roof. Something is causing water to get into the house through the system which could eventually damage the inducer, gas valve and burner assembly. This is what caused the port to be clogged. Replaced customer supplied air filter (Merv 11 16x25x5) that was overdue. Filter was changed last year 7/2021. Filter should be replaced once every 6-12 months. Inspected all the electrical components. Confirmed proper high and low voltages. Cleaned the burners, flame sensor and burner compartment. System had a temperature rise of 48°. Performed a combustion analysis on the system. System is operating at 83.8% efficiency with a CO