York heat pump in heating mode shows H1 and stops heating

Safety: Turn off power to the outdoor unit before servicing.

• Check for heavy frost: Inspect the York heat pump outdoor coil. If heavily iced, airflow or defrost control is suspect.
• Inspect defrost sensor: Locate the defrost thermistor on the coil. Ensure it is tightly attached and not corroded.
• Test sensor: Measure resistance at known temperatures and compare to York data. Replace if readings are incorrect.
• Check defrost board: Inspect the defrost control board for burned components or water damage. Verify correct wiring and dip switch settings.
• If unsure: Have a technician test the defrost cycle and replace the defrost board if necessary.