Ruud air conditioner outdoor unit shows H0; unit may not start or may run in a limited mode.

Safety first: Turn off power at the outdoor disconnect before opening the control panel.

• Step 1 – Identify if unit is a heat pump: H0 typically applies to Ruud heat pump condensers. Confirm model number indicates heat pump.
• Step 2 – Inspect defrost board: Locate the defrost control board in the outdoor unit. Check for loose connectors, corrosion, or burn marks.
• Step 3 – Check defrost sensor: The defrost sensor (coil thermistor) is usually clipped to the outdoor coil tubing. Ensure it is attached and wiring is intact.
• Step 4 – Reversing valve coil: Inspect the reversing valve solenoid coil for secure wiring and signs of overheating. A failed coil can cause mode errors.
• Step 5 – Technician service: If H0 persists after basic checks, a technician should test the defrost board, sensor resistance, and reversing valve operation, replacing faulty components as needed.