American Standard heat pump shows H0 and will not enter defrost or heat properly

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

• Inspect defrost sensor: Locate the defrost/coil sensor clipped to the outdoor coil tubing. Ensure it is firmly attached and the wires are intact.
• Check board connections: Open the control panel and verify that all connectors on the defrost board are fully seated. Look for burnt spots or corrosion on the board.
• Observe operation: Restore power and monitor the unit in heat mode. If the coil ices up heavily and the unit never enters defrost, or H0 appears, the board or sensor may be faulty.
• Replace defrost board if needed: If wiring and sensor are good, replacing the American Standard defrost control board is typically required and is best done by a technician to ensure correct setup.