Upon arrival, thermostat is staisfied and system is not running. Performed a pre-operation inspection of the furnace that showed a clogged internal drain trap and rusty burners. These both contribute to operational issues as after longer periods of run time, the drain trap being clogged will cause the system to shut down due to internal flooding. Rusty burners contribute to starting procedure errors as the burns could not properly light, and/or a flame sense signal not being transmitted. Performed a test fire off to test hypothesis for rusty burner issues. During fire off, the first burner with the ignitor ignited immediately, while the other burners had a delayed ignition due to burner condition. Blow out drain trap to remove any clogged debris and confirmed proper drainage. Removed and wire brush cleaning of burners to clear out rust buildup in burner connection channels. Inspection also presented that the furnace and AC systems are both two stage while the current thermostat is on
