Goal: Test thermostat operation, verify control system wiring, and diagnose 24V AC control circuits
Check thermostat power and basic settings:
- Examine the thermostat display - should show current temperature and be responsive
- Replace thermostat batteries if display is dim or unresponsive
- Set thermostat to COOL mode and temperature 5°F below current room temperature
- Expected result: Display shows clear readings and responds to setting changes
Remove thermostat from wall plate for wiring inspection:
- Carefully pull the thermostat body straight out from the wall plate
- Identify the wire colors and their terminal connections
- Look for loose connections, corrosion, or damaged wire insulation
- Expected result: All wires securely connected with no visible damage
Test 24V AC power supply with multimeter:
- Set your multimeter to AC voltage mode (V~)
- Test voltage between R wire (red, power) and C wire (blue/black, common)
- Should read 24V AC - this is the control voltage from the transformer
- Expected result: Multimeter reads 22-26V AC between R and C terminals
Test cooling system control circuit:
- With multimeter still on AC voltage, test between R wire and Y wire (yellow, cooling)
- Temporarily connect R and Y wires together using a wire nut to bypass thermostat
- Listen for outdoor unit to start running - this tests the cooling contactor
- Expected result: Outdoor unit starts when R and Y are connected, stops when disconnected
Test fan control circuit:
- Test voltage between R wire and G wire (green, fan)
- Connect R and G wires together to test blower fan operation
- Indoor blower fan should start running when wires are connected
- Expected result: Blower fan operates when R and G are connected
If no 24V power at R-C terminals: Check transformer fuse in indoor unit - may be blown
If outdoor unit doesn't start with R-Y connected: Problem is in contactor or compressor circuit
If blower fan doesn't run with R-G connected: Check blower motor capacitor and motor windings
You'll know you're done when: Thermostat and control circuits are fully tested and diagnosed
- Thermostat display is clear and responsive
- 24V AC power confirmed between R and C wires
- Cooling circuit tested by connecting R-Y wires
- Fan circuit tested by connecting R-G wires
- All wire connections are secure and properly terminated
Ready for next step? You've verified the control system works and can now test the electrical components that receive these control signals.