U4 - Overheat protection activated in outdoor unit / Outdoor thermistor short/open

What does U4 mean?

This code means the outdoor unit's overheat protection mechanism has been triggered, or the outdoor thermistor is either short-circuited or open-circuited.

• Blocked airflow to the outdoor unit (dirty coils, obstructions).
• Malfunctioning outdoor fan.
• System running under high ambient temperatures.
• Faulty outdoor thermistor sensor.
• Loose or damaged wiring to the outdoor thermistor.

Click steps to track your progress.

1. 1

   Clear any debris, leaves, or obstructions from around the outdoor unit to ensure proper airflow.

2. 2

   Check that the outdoor fan is spinning freely and not making unusual noises.

3. 3

   Turn off the system at the circuit breaker for 30 minutes to allow it to cool down and reset.

4. 4

   If the error persists, contact a qualified technician to check the outdoor thermistor and its connections.