Ductless systems fail differently from central air. The indoor unit holds its own coil, fan, drain and electronics; the outdoor unit runs an inverter compressor controlled by a circuit board; and the two talk to each other over a single cable. When a mini-split in a San Jose home stops cooling or heating, the cause is usually in one of those small places, and the system often reports it with blinking lights or a code on the display.
The problems we see most
Not cooling or heating
The usual causes are clogged filters, an iced indoor coil, low refrigerant from a leaking flare joint, a failed temperature sensor, or a fault in the outdoor fan or inverter board. Multi-zone systems add one more: when one indoor unit is set to heat, the others usually cannot cool at the same time, so the rooms set to cool sit idle or show a mode warning.
Error codes and blinking lights
Every manufacturer has its own codes, and one code can point to several causes. Communication faults are common: a loose terminal, a damaged cable between the units, or a board that no longer sends its signal. Write down the pattern and which unit shows it before you call.
Water leaks
Water dripping from the indoor unit usually comes from a clogged drain line, a failed condensate pump, a unit that has shifted out of level, or a coil that froze and is now thawing. Switch the unit off and catch the water.
Remote control problems
Plenty of units that seem dead have a remote problem: flat batteries, the mode left on Dry or Fan, or a locked remote. A wired wall controller can also lose contact with the unit. A lost or broken remote can be replaced.
Strange odors
A musty, sour smell comes from mold and bacteria on a damp coil and blower wheel. Washing the filters helps for a while; cleaning the coil and the wheel removes the source. A burning smell is different: switch the unit off at the breaker and call.
How a ductless system is diagnosed
- Read the fault. The current code, and on many systems the stored fault history, narrows the search.
- Check what restricts it. Filters, coil, blower wheel and drain.
- Test power and communication. Supply voltage at both units, and the cable and terminals that connect them.
- Test the sensors. Room, coil and outdoor temperature sensors are checked against their specifications. A failed sensor costs far less than the board it is often mistaken for.
- Check the refrigerant last. If a leak is suspected, the joints are tested and the leak repaired; the system is then pressure tested, evacuated and recharged by weight. Refrigerant work is done by EPA-certified technicians.
A circuit board is replaced only after the sensors and wiring have been ruled out.
What you can check before calling
- Filters. Lift the front panel, slide out the mesh filters, rinse them and let them dry completely.
- Remote. Fresh batteries; mode on Cool, Heat or Auto; a setting well away from room temperature.
- Power. The breaker is on and the outdoor disconnect is in place.
- Winter steam. On cool, damp nights the outdoor unit may stop and release a cloud of steam for a few minutes. That is normal defrost. A unit that stays caked in ice is not, and our page on heat pump freezing explains why.
Do not open the outdoor unit’s electrical cover: inverter boards can hold a charge after the power is off.
San Jose notes
Many mini-splits here were added to converted garages, room additions and older Willow Glen and Rose Garden homes without ducts. Three problems come up again and again:
- Hot side yards. An outdoor unit squeezed into a narrow, sun-baked side yard breathes its own hot air on the hottest afternoons and can shut down on high pressure. Sometimes the fix is clearance, not parts.
- Line insulation. The foam on the refrigerant lines cracks in the sun outside, and where it fails on lines running through a hot attic the system loses capacity.
- Dust. Dry summers load the filters fast, and units near a kitchen collect grease on the coil.
Cost, and repair or replace
Most mini-split repairs fall between $150 and $800. Sensors, drain clearing and cleaning sit at the low end; leak repair, fan motors and control boards cost more. You approve the price before any work starts, and the diagnostic fee is waived if you go ahead. Same-day service is available for most calls received before noon.
Repair usually wins when the system heats and cools well apart from one failed part. Replacement is worth pricing when an older outdoor unit needs a compressor, when the same joint leaks again, or when parts for a discontinued model are hard to find. If that is where you are, see mini-split installation. Central heat pump systems are covered under heat pump repair.