Performed an A/C tune-up on two Carrier heat pump systems. Checked the electrical, mechanical and performance systems of the unit. Cleared, cleaned, flushed and treated the drain line and washed the outdoor coils. All filters were replaced recently by homeowner and are in clean condition. I then performed diagnostic on the downstairs south system and found the thermostat has failed. I spoke with homeowner and he chose to have the thermostat replaced. Installed a new thermostat for the downstairs south system. Checked operation of the system and completed the tune up.