Performed furnace clean and tune; started by inspecting the exhaust and intake pvc ensuring all connections were secured, I then removed and cleaned burners along with the flame sensor, I then cycled the system on and checked gas pressures did find second stage not engaging after investigating did find the Tstat being a single stage Tstat, I replaced this Tstat with a T6 I cycled the sytem on and got second stage to engage, lastly I performed a combustion test verifying no CO in the airstream. Job complete