Performed pm on oil furnace Brushed and vacuumed flue pipe and heat exchanger Replaced oil filter, nozzle, and strainer Checked all electrical connections and safeties Performed combustion and smoke tests 10.2%o2 0ppm co 0 smoke While performing the maintenance the customer stated they were having issue with the second floor thermostat. The display was not coming on. Took the thermostat from the first floor and tested it on the second floor subbase. With the thermostat switched it powered on indicating the thermostat is faulty. After getting customer authorization we replaced the second floor thermostat with a new one. Programmed the thermostat and tested operation. Unit is working properly