HVAC Error Code U6 — What It Means and How to Fix It

What Does This Error Mean?

HVAC error code U6 means the system detected that the compressor is drawing too much electrical current — a compressor overcurrent fault. The system shuts down automatically to protect the compressor from burning out. U6 is a serious error that almost always requires a professional technician. Ignoring it or repeatedly resetting the unit risks permanent compressor failure.

Affected Models

Mini-split heat pumps
Ductless AC systems
Inverter compressor systems
Mitsubishi split systems
Daikin split systems
Carrier mini-splits

Common Causes

The compressor is mechanically worn or partially seized, causing it to work harder and draw more current than normal
The power supply voltage is too low (a brownout condition), forcing the compressor motor to draw extra current to compensate
The outdoor unit's fan has failed, causing the compressor to overheat and work harder to maintain pressure
The refrigerant system is overcharged or has a blockage, putting excessive mechanical load on the compressor
The compressor's electrical windings are partially short-circuited internally, causing abnormal current draw

How to Fix It

Turn off the unit at the thermostat and then at the circuit breaker. Do not attempt to restart it until the cause of the overcurrent has been identified and fixed.

Each time you restart a unit with a compressor overcurrent condition, you shorten the remaining life of the compressor. This is a stop-running situation, not a try-again-and-see situation.
Check that the circuit breaker for the outdoor unit is the correct amperage rating. Look at the nameplate on the outdoor unit for the Maximum Overcurrent Protection (MOP) rating and compare it to your breaker.

An undersized breaker will trip frequently, but an oversized breaker may allow too much current to flow and cause overheating. The breaker should match the unit's specification within a few amps.
Check for obvious power supply problems. If lights in your home are dimming or other appliances seem sluggish, you may have a brownout condition from the utility. Call your electricity provider.

Low supply voltage forces all motor-driven appliances to draw more current than normal. This is a common cause of U6 during summer heat waves when everyone is running their AC simultaneously.
Check that the outdoor condenser unit's fan is spinning. With the power OFF, gently try to rotate the fan blade by hand. It should spin freely with no resistance.

A seized outdoor fan motor forces the compressor to work against excessive pressure, which increases its current draw. Fan motor replacement is a standard repair that is much cheaper than replacing the whole compressor.
Call a licensed HVAC technician to test compressor current draw, check refrigerant charge, and inspect the power supply wiring. This is not a DIY repair.

The technician will use a clamp meter to measure running current and compare it to specifications. If the compressor is internally failing, they can advise whether repair or replacement is more cost-effective.

When to Call a Professional

U6 is a critical error that requires a licensed HVAC technician with electrical testing equipment. The technician will use an ammeter to measure actual compressor current draw and compare it to the nameplate specification. Do not keep resetting and restarting the unit — a compressor that is drawing too much current will eventually burn out its internal windings, making full compressor replacement necessary. Call a technician as soon as possible.

Frequently Asked Questions

How do I know if my compressor is failing or if it is just a power issue?

A technician can tell the difference with a clamp meter. If the current draw is high but normalizes when supply voltage is corrected, the power supply was the problem. If the current remains high even with correct voltage, the compressor itself is struggling. A compressor with failed internal windings will also show abnormal resistance readings when measured with a multimeter. This is straightforward for a technician but requires specialized tools.

Is a U6 error always fatal for the compressor?

Not always. If caught early, U6 might indicate a repairable cause — like a failed fan motor or a power supply issue — rather than the compressor itself. The earlier you stop the unit and call a technician, the better the chance of avoiding full compressor failure. If the unit has been repeatedly restarting through overcurrent faults for weeks, the compressor windings may already be damaged.

What does it cost to replace an HVAC compressor?

Compressor replacement is one of the most expensive HVAC repairs. For a mini-split system, a replacement compressor typically costs $400 to $1,500 for the part, plus $300 to $600 for labor. For a full central air conditioning system, it can be $1,000 to $2,500 or more. Because of this cost, technicians often recommend replacing the entire outdoor unit rather than just the compressor if the unit is more than 8 to 10 years old.