Gree Split AC Error Code U4 — What It Means & How to Fix It

What Does This Error Mean?

U4 means the system detected the compressor spinning in reverse during operation. This is similar to U2 but triggered while running, not just at startup. Phase wiring error or a compressor motor fault are the main causes.

Affected Models

Gree FAIRY Series
Gree LOMO Series
Gree U-MATCH Series
Gree PULAR Series
Gree Inverter Split System

Common Causes

Compressor phase wiring connected in wrong order — same root cause as U2
Drive board losing synchronisation with the compressor motor during operation
Compressor motor bearing or mechanical fault causing erratic rotation
Drive board fault sending incorrect phase sequence after a voltage event
Low refrigerant causing compressor to hunt and lose rotor synchronisation

How to Fix It

Turn off the AC and switch off the outdoor unit circuit breaker.

The unit has already protected itself by shutting down — confirm power is fully removed.
Do not restart the unit until the cause has been identified.

If this is a phase wiring issue, restarting will cause the same problem and progressive damage.
Check whether you have also seen U2 errors — if so, phase wiring is almost certainly the cause.

U2 at startup and U4 during operation both point to the same wiring fault.
Call a Gree-certified technician and describe both error codes if U2 was also seen.

The technician will check the phase wiring order first, as this is the quickest fix to rule out.
If wiring is correct, the technician will check the drive board output and compressor motor condition.

A drive board losing sync with the compressor can also cause U4 and may require board replacement.

When to Call a Professional

Call a technician to diagnose U4. Reverse rotation at operating speed causes severe and rapid compressor damage. The unit shuts down immediately when U4 is detected — do not attempt to restart without professional diagnosis.

Frequently Asked Questions

What is the difference between Gree U2 and U4 errors?

U2 is a phase detection fault found at startup — before the compressor actually runs. U4 is a reverse rotation fault detected during operation. Both can have the same root cause (wrong wiring phase order) but U4 can also indicate a drive board sync problem.

How quickly does reverse rotation damage a compressor?

Reverse rotation at operating speed can damage the compressor valve system within seconds. Gree units detect this quickly and shut down immediately. If the protection triggered promptly, compressor damage may be minimal — but a technician must confirm.

Can U4 appear after a refrigerant top-up?

In rare cases, yes. If refrigerant was added incorrectly and the system is overcharged, the compressor load changes dramatically. The drive board may temporarily lose synchronisation with the compressor rotor and trigger U4. Refrigerant levels should be checked after any top-up.