Mini-Split Not Cooling or Heating: A Diagnostic Order for Ductless Systems
Mini-splits fail differently from ducted systems. The electronics do a lot of self-diagnosis, the charge is small and precise, and a dirty blower wheel can cut capacity in half.
Safety: Inverter boards hold dangerous voltage after power is off. See working safely on mini-split inverter boards.
1. Read the code
Check the display or the LED blink pattern, and look up the code for that brand and series in our mini-split error codes. Common categories:
- Communication errors between indoor and outdoor units. See mini-split communication errors.
- Sensor (thermistor) faults. See how to test a mini-split thermistor.
- Protection codes: high or low pressure, high discharge temperature, overcurrent, which often point to charge, airflow or a dirty coil.
2. Settings and the remote
- Mode set correctly (COOL, HEAT, not FAN or DRY)
- Setpoint well below room temperature in cooling, above in heating
- Remote batteries; the head beeps when it receives a command
- Timers, quiet or eco modes, and any central controller or Wi-Fi schedule overriding the remote
3. Airflow at the indoor unit
- Filters: often clogged on units that run constantly.
- Blower wheel and coil: a dirty wheel cuts airflow badly and can make the head spit water. See how to clean a mini-split.
- Louvers stuck closed or pointed at a wall.
4. Is the outdoor unit running?
| Outdoor unit | Check |
|---|---|
| Completely dead | Power at the disconnect and terminal block; fuses on the outdoor board |
| Powered, won't start | Communication wiring between units; board LEDs; inverter faults |
| Fan runs, compressor doesn't | Compressor protection codes, inverter output, compressor windings |
| Runs, then stops after a few minutes | Protection codes for pressure or temperature; charge; dirty outdoor coil |
Many mini-splits wait several minutes after power-up or a mode change before starting the compressor. Give it time before calling it dead.
5. Performance problems with everything running
- Temperature split across the indoor coil
- Outdoor coil clean and clear; in heating, defrost working
- Thermistors reading correctly against a thermometer
- Charge: most manufacturers want mini-splits charged by weight. If you suspect low charge, leak check (flares first), recover, repair, evacuate and weigh in the nameplate charge plus any line set adjustment. See mini-split line set length and charge and flare connections.
Common mistakes
- Connecting gauges first. Many mini-splits have only one service port, and every connection loses a little refrigerant.
- Topping off by pressure instead of finding the leak and weighing in.
- Missing the blower wheel because the filters look clean.
- Replacing a board for a communication error that's really a wiring or connection problem.
- Not waiting for the startup delay.
FAQ
Why is my mini-split running but not cooling?
Dirty filters or blower wheel, the wrong mode, a dirty outdoor coil, a low charge from a flare leak, or a sensor problem. Check the error code and airflow first.
How do I reset a mini-split?
Turn off power at the disconnect for a few minutes, then restore it. That clears many codes, but the cause comes back if it isn't fixed.
Do mini-splits need refrigerant added?
Not normally. They're sealed systems; low charge means a leak, most often at a flare connection.
Sources
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Related guides
How to Clean a Mini-Split: Filters, Coil, Blower Wheel and Drain
How to deep clean a ductless mini-split indoor unit, including the coil, blower wheel and drain, plus the outdoor unit, with the right tools and without damaging the electronics.
Mini-Split Communication Errors: How to Troubleshoot Indoor-to-Outdoor Faults
A step-by-step approach to mini-split communication faults between the indoor and outdoor units, covering wiring, polarity, voltage and boards.
How to Test a Mini-Split Thermistor (Temperature Sensor)
Test a mini-split room, coil, discharge or outdoor temperature sensor with an ohmmeter and the manufacturer's resistance table, and find out whether the sensor, wiring or board is bad.
Working Safely on Mini-Split Inverter Boards
Why mini-split inverter boards stay dangerous after power-off, how to verify the DC bus is discharged, and how to approach IPM, voltage and compressor drive error codes.
Field reference for trained technicians. Always follow the equipment manufacturer's instructions, local code and safe work practices. See our editorial standards, or report a correction.