Display shows A3, unit locked out, compressor stopped

Safety: Turn off power before removing any covers. Hot surfaces and high voltages are present.

• Check airflow and water flow: For air‑source Nibe units, ensure the outdoor coil is not blocked by leaves, snow, or debris. For water‑based systems, verify that the condenser circuit pump is running and that valves are open.
• Clean heat exchangers: Gently clean the outdoor coil fins with low‑pressure water (power off) and a soft brush. Do not bend fins.
• Inspect discharge sensor wiring: With power off, locate the discharge gas temperature sensor on the compressor discharge line and check for loose or damaged wiring.
• Reset alarm: Restore power and clear the A3 alarm via the control panel. Monitor operation; if the alarm recurs, stop the unit.

Call a technician if the alarm returns. A professional must check refrigerant charge, compressor performance, and replace the discharge temperature sensor if faulty.