Keystone air conditioner powers on but compressor does not start; fan runs and air is not cold.

Warning: Capacitors store high voltage even when unplugged. Discharge capacitors properly or have a professional handle this step.

Steps:

• Listen for compressor: When cooling is requested, you should hear a deeper hum from the compressor. If you only hear the fan, suspect a start issue.
• Inspect wiring: With power disconnected, remove the access cover over the compressor terminals. Check for loose or burnt wires and reconnect or replace as needed.
• Test capacitor: Locate the dual or single run capacitor (often metal can). Use a capacitance meter to verify microfarad (µF) value matches the label. Replace if more than 10% out of spec or bulged/leaking.
• Relay/board check: On PCB-controlled units, inspect the compressor relay for burn marks. If the relay is defective, replace the main control board or relay module.
• Compressor test: If capacitor and wiring are good but compressor still will not start or trips breakers, the compressor may be locked or internally failed and will require professional sealed system service.