No heat call. Started with clean and diagnosing of furnace. Customer wanted to join maintenance member plan. After diagnosing, found high limit switch was failed to the open position. The furnace filter was extremely dirty and clogged up, thus making the high limit fail. Also tested flame sensor, pressure switches, hsi, fan motor amperage and capacitor, and inducer motor amperage. Replaced the high limit switch and hsi due to corrosion. Everything else was in adequate working condition. One visit on plan was used today and customer has one remaining for spring for condensing unit. Performed free inspection.