Performed FAU maintenance on 5 year old Lennox unit. Cleaned return grate. Inspected filter, it is still in good condition. Cleaned condensate drain P trap and flushed with water. Tested external static pressure and found it to be high on the supply side. Inspected blower compartment, blower wheel is clean and balanced. Tested all safety switches. Inspected flame sensor. Tested resistance of hot surface igniter. Temperature rise is within limits. Checked accuracy of thermostat and found it to be within 1 degree. Upon departure system is operational.