HVAC Error Code P4 — What It Means and How to Fix It

What Does This Error Mean?

HVAC error code P4 typically indicates an inverter or compressor module fault. On most mini-split systems, P4 means the compressor's inverter drive detected an abnormal condition. This is a serious error that usually requires a technician to diagnose.

Affected Models

Mini-split heat pumps
Inverter-type AC units
Daikin split systems
LG split systems
Generic inverter AC units

Common Causes

The compressor is drawing too much current, indicating it may be failing
The inverter (IPM) module on the control board is overheating or faulty
Low refrigerant causing the compressor to work harder than normal
A wiring problem between the outdoor control board and the compressor
A power surge damaged the inverter module inside the outdoor unit

How to Fix It

Turn the unit off immediately and do not continue running it. Operating the system with a P4 fault can permanently damage the compressor or inverter board.

The P4 fault is a protective shutdown. Overriding it risks a much more expensive failure.
Turn off the AC's circuit breaker for 10 minutes, then turn it back on. Restart the unit and see if the error clears. Sometimes P4 is triggered by a temporary power spike.

A power cycle sometimes clears a false P4 alarm caused by a voltage surge. If P4 returns on the next cooling or heating cycle, the issue is real.
Check that the outdoor unit has adequate airflow. Clear any obstructions within 2 feet of all sides. A hot, restricted outdoor unit causes the compressor to overload and trigger P4.

Overheating is a common cause of inverter module faults. Good airflow around the outdoor unit is essential.
Check for signs of refrigerant leak: ice on the refrigerant lines, hissing sounds, or the unit blowing warm air before the error appeared. Low refrigerant forces the compressor to work harder.

Low refrigerant can trigger P4 because the compressor overworks trying to compensate. Only a technician with proper gauges can check refrigerant level.
Call a licensed HVAC technician for diagnosis. Explain when P4 first appeared and any observations (ice, sounds, warm air). They will use diagnostic tools to check the compressor, IPM module, and refrigerant charge.

Inverter board and compressor repairs or replacements are not DIY jobs. The parts are expensive and require professional installation.

When to Call a Professional

P4 almost always requires a professional HVAC technician. The compressor and inverter board are complex, high-voltage components. Attempting to repair them without proper training and tools is dangerous. A technician will measure compressor current draw, check refrigerant, and test the IPM module.

Frequently Asked Questions

What is an inverter in a mini-split AC?

In a regular AC, the compressor runs at full speed or stops. An inverter-type AC has a variable-speed compressor that can run at different speeds. This makes it much more energy-efficient. The inverter (IPM module) controls the compressor speed. When the inverter detects a problem, it shows P4 and shuts down to prevent damage.

How much does it cost to fix a P4 error?

Cost depends on the cause. If it is just a refrigerant issue: $150–$400 for a recharge. If the inverter IPM board needs replacement: $300–$800 for parts and labor. If the compressor has failed: $600–$2,000 or more. Sometimes it is more cost-effective to replace an older unit.

Does P4 always mean the compressor is broken?

Not always. P4 can be triggered by low refrigerant, a power issue, or an overheating inverter board — all without actual compressor damage. A technician can diagnose which component is at fault. Catching P4 early and not continuing to run the unit gives the compressor the best chance of survival.