Safety warning: Turn off power to both indoor and outdoor units before working on wiring.

Step 1 – Verify thermostat call: Set the thermostat to COOL and lower the setpoint. Confirm that the thermostat indicates a cooling call is active.
Step 2 – Check Y voltage at thermostat: With a multimeter and call active, measure between Y1 and C at the thermostat. You should see ~24 VAC. If not, the thermostat output may be faulty.
Step 3 – Check Y voltage at indoor board: Measure between Y and C at the indoor control board. If voltage is present here but not at the outdoor unit, the cable to the outdoor unit may be open.
Step 4 – Inspect outdoor low-voltage wiring: At the outdoor unit, check the Y and C connections. Tighten terminals and repair any damaged cable.
Step 5 – Inspect outdoor fuse and board: Some Trane outdoor units have low-voltage fuses or resettable breakers. Check and reset/replace as needed.
Step 6 – If wiring and voltages are correct: The outdoor control board or contactor may be faulty and should be tested by a technician.

Thermostat calls for cooling but outdoor unit does not start

Possible Causes