Mini Split Failures, Problems & Troubleshooting Guide

The most common cause of a mini split failure is improper installation. Whether you are experiencing an issue with your unit or just trying to learn how to do it right the first time around, this guide will take you through the most common problem with a mini split.

Mini Split Installation

Communication Issues

Using the proper mini split communication wire is critical to a properly functioning unit. While some manuals may only call for 18 gauge wire it is always recommended to use 14 gauge 4 conductor wire (14/4) for most systems. Most local codes require 14 gauge wire, so using something smaller will not pass inspection in many areas. The wire should be stranded, not solid core. Solid core wire has been known to cause more issues. Make sure the wire you select is rated for indoor and outdoor use. If you are installing a unit in a commercial application, an armored cable is usually required. As with anything, check your local electrical codes and consult an electrician to ensure proper installation.

Improper wiring can lead to the following common problems:

  • Poor Signal. Symptoms of poor signal include:
    • Inconsistent operation
    • Unit working in some modes, but not others
    • Outdoor unit powers up but indoor unit does not
    • Indoor unit has power, but will not operate
  • Damaged or destroyed boards or compressor
    • Improper wiring can cause other system components to fail and require replacement.
  • Odd or loud noises
  • Communication error codes being displayed
    • Error codes may be visible on the indoor unit or outdoor board
    • Look for a series of lights blinking or a special code displayed

Grounding Issues

Grounding is important for any device that contains electronics. An air conditioner is no different. The control boards used in a modern mini split system are very much a miniature computer. These boards are susceptible to power surges, stray electrical energy, and RF (radio frequencies). Grounding the outdoor unit helps prevent some of the stray energy from getting into the board.

If your condenser is not grounded properly, you may experience some issues, such as improper operation. Worst case, you can have board and/or compressor failure. Improper grounding can cause the same problems as no ground.

When wiring your system, make sure the connections are correct. Most systems will have a terminal block with L1, L2 and C. L1 and L2 are your hot lines. C is your common or ground. You can add a fourth wire to add an additional ground, which never hurts. Grounding the chassis of the condenser to a ground rod also adds an extra level of protection. If you use a grounding rod, check it on occasion for deterization.

If you are unsure about grounding, consult with an electrician.

Mini Split Line SetsRefrigerant Issues

All modern mini split systems use R-410A. This refrigerant is much more environmentally friendly than the older R-22. It operates at a much higher pressure than other refrigerants and requires special care to ensure a properly functioning system. Make sure that you are using the right gauges for R-410A.

Most systems come precharged with some refrigerant, but depending on the length of your line set, more might need to be added. The installation manual for your unit will specify the weight of refrigerant needed per foot of line set. Always follow these guidelines exactly. An improper amount of refrigerant can lead to many issues, including:

  • Too little of a charge
    • System may not run at all and may show an error code
    • The system may produce loud noises
    • It might not be able to maintain the temperature in the space
    • It might not properly dehumidify
    • It might only function in cooling mode but not in heating mode
  • Too much of a charge
    • System may not turn on
    • Loud noises can be heard from the unit.
    • Unit may not be able to hold the temperature or may overshoot the setpoint

The use of a proper line set is also critical to system performance. Each mini split system will specify the proper size line set it needs including the size of the pipes, the minimum and maximum length, and the maximum height difference between the indoor and outdoor unit. Trying to reuse an old line set that is not correct for your system will result in failure.

Line sets must be insulated. Unlike a central system, both lines require insulation. At least ½” insulation must be used to ensure proper performance. Make sure the entire line is insulated all the way up to the flair connections. No bare copper should be visible.

Always make sure to completely vacuum the lines before introducing any refrigerant into the system. Any contaminants in the lines could lead to severe damage to internal components or the system not working at all. It’s also important to pressure test the system. This will make sure that there are no leaks.

Live Voltage Issues

The vast majority of mini split units require 208/230 volt service, but there are some that will work with 115 volts. Always make sure that your system is connected to the proper voltage. Even if the system is correctly wired to the proper voltage, issues can still occur depending on your electric service. If too high or too low of a voltage is delivered to the unit, damage to the boards can occur. Check the voltage at the unit before starting up the system and make sure it is within the approved range that the manufacturer states. If your voltage is too high, too low, or often fluctuates you should use a surge protector to make sure your system components are protected.

Water Leaking at the Indoor Unit

If you discover water leaking from the evaporator of your mini split there are several areas to check to determine the cause of the problem.

If you have a wall mounted indoor unit, the drain line must be pitched down at least 5 degrees when leaving the unit and going outside. Otherwise, the water can drip back into the room. If the the drain line is properly pitched, then the next step is to check for clogs in the line. You can use a gallo gun to clear any obstructions in the drain line.

A condensate pump can be used in installations where you are not able to drain with gravity. If you have a condensate pump and are experiencing leaking, check to make sure the condensate pump is properly wired and functioning. Make sure that the pump is installed properly and is not pumping higher than its rated height.

Environmental Issues

The temperature outside can impact a mini splits performance. If you have a heat pump unit and are trying to use it in cold weather, there are a few steps you can take to make it perform better.

If your unit has an optional drain pan heater, adding this can increase the temperature range in which the unit will operate. A wind baffle can also provide some help. Make sure that if you are in a location that gets snow your unit is installed on a stand or wall mount that is above the snow. You should also remove any snow from the top of the unit before operating. Overly windy areas can also negatively impact the unit.

If you are trying to use cooling at low temperatures for server room or other environments that call for cooling all year long, make sure you purchase a unit that offers low ambient cooling. A wind baffle will expand the range of the unit. Some system are designed with extreme ranges in mind. Make sure you check your units specifications when looking for all year cooling.

Repair or Replace?

If you have just purchased a new unit and are experiencing any of the issues outlined here, call your installer immediately to get them sorted out. If you are having trouble with an older mini split, it may be time for a new one. Replacing boards or compressors on older systems can be expensive and time consuming.

LG and Daikin are the leaders when it comes to reliable, long lasting mini split units. Exceptional build quality and commercial grade components are backed by industry leading warranties. Let our experts help you choose the best new mini split for your application.

Mickey Luongo

Mickey is the resident heating and air conditioning expert.



about 2 months ago

Replaced both capacitors and mother board in condenser ...split ac unit w/one head unit inside. Still getting blinking red light and pulsating/flickering green light. Condernser will not come on...head unit opens and closes flap, but no fan! Thanks, Harry



about 2 months ago

where to I find the reset button unit not responding to remote control


Mickey Luongo

about 2 months ago

Most mini splits do not have a reset button.



about 1 month ago

Hi, Our older unit (Mitsubichi mini split) suddenly will not heat. Everything else works well, fan cooler, dehumidifier but when we set it to heat, it turns off.


Chuck Purce

about 6 days ago

During the heating mode, Is it normal for the condenser on the unit to have accumulated frost all around the exposed condenser? The weather overnight was in the 20’s.


Mickey Luongo

about 6 days ago

Depending on the unit, this is possible. Some mini splits, especially older ones do not perform well in cold temperatures. If available for your unit, you may want to add a drain pan heater or wind baffle.


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