Mini-split not cooling or heating: guided troubleshooting
Ductless unit that won't run, or runs without conditioning the room. Answer each question from what you see on site.
Step 1
Is the indoor unit showing an error code or blinking lights?
See every step in this flow
- Is the indoor unit showing an error code or blinking lights?
- Yes: Look up the code
- No: Does the indoor unit power up and respond to the remote?
- Does the indoor unit power up and respond to the remote?
- Yes: After a few minutes in the right mode, is the outdoor unit running?
- No, it's dead: No power at the indoor unit
- After a few minutes in the right mode, is the outdoor unit running? Inverter units ramp up slowly and may wait several minutes before starting.
- Yes: Are the indoor filters and coil clean, and is air moving freely?
- No: Outdoor unit isn't starting
- Are the indoor filters and coil clean, and is air moving freely?
- No, dirty or blocked: Clean it first
- Yes: Was the system recently installed, or has it been losing capacity over time?
- Was the system recently installed, or has it been losing capacity over time?
- New install that never worked right: Check the installation
- Worked well, now getting weaker: Likely losing charge
- Stopped suddenly: Sensor, valve or board
Where each path ends
Look up the code
The code names the circuit. Look it up for the brand and series before testing.
No power at the indoor unit
Most indoor units are powered from the outdoor unit through the interconnect cable. Check the outdoor disconnect and breaker, then the voltage and wire order on the interconnect terminals at both ends.
Outdoor unit isn't starting
Check the mode and setpoint, then power and communication at the outdoor unit. Inverter boards hold a dangerous charge after power-off; wait and verify before touching them.
Clean it first
A matted filter, coil or blower wheel cuts capacity sharply on a ductless unit. Clean, then re-test.
Check the installation
Look at flare connections, line set length against the factory charge, and whether a proper vacuum was pulled.
Likely losing charge
Flare connections are the usual leak point. Find and fix the leak, then recover, evacuate and weigh in the nameplate charge. Mini-splits are charged by weight.
Sensor, valve or board
With no code showing, check the thermistors against the manufacturer's resistance table, then the reversing valve and the outdoor board.