What usually fails, and what the repair involves

What you see

Leak alarm code or fast-flashing LED on the detection board; cooling and heat pump heating are locked out and the indoor blower runs. Whether electric heat is switched off depends on the brand. Some ductless units also sound a buzzer.

Usual cause

The sensor is detecting refrigerant, meaning a leak in the indoor coil section; or, per one manufacturer, a faulty sensor.

What the repair involves

The alarm is treated as a real leak first: the system is leak checked, the leak is repaired and the charge restored. If no leak is found, the sensor itself is evaluated. The system returns to normal about five minutes after the alarm clears.

One manufacturer's manual instructs: if the leak alarm is observed, do not open the unit or turn it off.

Sources: EPA SNAP Rule 23 — 86 FR 24444 · Daikin / Goodman service manual RS6200301 (A2L) · Bosch ductless installation and service manual

What you see

Detection board flashes a fault pattern, or the control shows a sensor-fault code; cooling and heat pump heating are inhibited although no refrigerant is leaking.

Usual cause

The sensor reports an internal fault, the board cannot communicate with the sensor (sensor, harness or board), or the board has an internal fault. One manufacturer also lists a faulty sensor as a possible cause of a leak alarm itself.

What the repair involves

The sensor connection is reseated and power is cycled. If the fault remains, the sensor, the control board, or both are replaced as the manufacturer's fault table directs.

Sources: Daikin / Goodman service manual RS6200301 (A2L) · Bosch ductless installation and service manual

What you see

Sensor error code, or the sensor has reached the service life the manufacturer states.

Usual cause

The sensor is a life-limited part that cannot be repaired. One ductless manufacturer states a 15-year service life; no figure common to all brands was found.

What the repair involves

The sensor is replaced only with the part the equipment manufacturer specifies, mounted on its original bracket in its original position. The detection system that the indoor unit was approved with is kept as built.

One ductless manufacturer states a 15-year sensor service life. No figure common to all brands was found.

Sources: Bosch ductless installation and service manual · Daikin / Goodman service manual RS6200301 (A2L)

What you see

Thermostat is calling but the compressor does not start or trips on its protector; a compressor diagnostic module reports a locked-rotor or open-start-circuit code. The capacitor may be visibly bulged, dented or leaking.

Usual cause

Capacitor is open, internally shorted, or its capacitance has fallen outside the tolerance printed on it. The manuals describe how a failed capacitor tests, not why it failed.

What the repair involves

The capacitor is discharged, tested and, if open, shorted, visibly damaged or out of tolerance, replaced. Power off; the part stores a charge.

Sources: Daikin / Goodman service manual RS6200006 · Carrier service manual 24-25-2SM

What you see

Thermostat is calling and control voltage is present, but the compressor and outdoor fan get no line voltage (contactor failed open); or the compressor has voltage with no call for it (contactor stuck closed).

Usual cause

Burned or pitted contact points, an open holding coil, or contacts stuck closed. DOE attributes control wear to frequent on/off cycling and corrosion of wires and terminals.

What the repair involves

Coil and contacts are checked with a meter; a contactor that will not pull in or does not pass voltage is replaced.

Sources: U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · Carrier service manual 24-25-2SM · Daikin / Goodman service manual RS6200006

What you see

Outdoor fan is not running while the compressor is; the system trips on high pressure.

Usual cause

Motor winding open or grounded, or the motor's auto-reset overload has opened on high temperature or high amperage.

What the repair involves

After the overload has had time to reset, the windings are checked for continuity and for a short to the frame; a failed motor is replaced.

Sources: Daikin / Goodman service manual RS6200006 · Carrier service manual 24-25-2SM

What you see

Compressor tries to start, draws locked-rotor current and cycles off on its internal protector; lights may dim on start.

Usual cause

Failed run capacitor, low line voltage (including long runs of undersized wire), excessive liquid refrigerant in the compressor, or seized bearings.

What the repair involves

Voltage during start, capacitor and wiring are checked first. Where starting voltage is low, a manufacturer-approved hard start kit may be added. If the compressor still draws locked-rotor amps with every external cause eliminated, it is replaced.

Sources: Daikin / Goodman service manual RS6200006 · Carrier service manual 24-25-2SM

What you see

Compressor does not run although power and controls check out, or the breaker/fuse trips when it tries to run.

Usual cause

Open or grounded motor windings. A burnout decomposes refrigerant, oil and insulation into acid and sludge. DOE notes that dirty filters and coils can lead to premature compressor failure.

What the repair involves

Windings are tested for continuity and for a ground. If the compressor has failed, the refrigerant is recovered, the compressor and filter driers are replaced (with an acid clean-up if the oil tests acidic), and the system is evacuated and recharged.

Sources: U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · Daikin / Goodman service manual RS6200006

What you see

System will not change between heating and cooling, or runs with abnormally high suction pressure and poor capacity that looks like a weak compressor.

Usual cause

Solenoid coil open or not receiving 24 V; pilot plunger stuck; valve stuck in mid-position or leaking internally from discharge to suction.

What the repair involves

Control voltage and coil continuity are checked first; an open coil is replaced on its own. If the coil is good and powered but the valve will not shift, the valve itself is replaced, which means recovering the charge and brazing.

Replacing only the solenoid coil does not open the refrigerant circuit; replacing the valve does.

Sources: Daikin / Goodman service manual RS6200006

What you see

In heating, the outdoor coil never defrosts; or the unit runs a defrost cycle at every interval whether or not the coil is frosted.

Usual cause

Defrost thermostat stuck open (no defrost) or stuck closed (defrost every interval), or a defrost control board that does not sequence the reversing valve, outdoor fan and auxiliary heat.

What the repair involves

A defrost cycle is forced with the board's test pins and the outputs are checked; the defrost thermostat is checked against its open/close temperatures. Whichever part fails the test is replaced.

Sources: Carrier service manual 24-25-2SM · Daikin / Goodman service manual RS6200006

What you see

Reduced capacity and efficiency; high head pressure and pressure trips in cooling.

Usual cause

Dust, pollen, vegetation and clutter on or around the coil; aluminum fins bent so they block airflow.

What the repair involves

With power off, debris and vegetation are cleared from around the unit. Cleaning the coil itself and straightening bent fins with a fin comb is contractor work.

Owner: clearing debris and vegetation, with power off. The coil and the fins are for a contractor.

Sources: U.S. DOE Energy Saver — Operating and Maintaining Your Heat Pump (archived copy) · U.S. DOE Energy Saver — Air Conditioner Maintenance (archived copy) · Daikin / Goodman service manual RS6200006

What you see

Usually none the owner sees; found on inspection as a heater that tests open. The consequence is liquid refrigerant in the compressor at start-up.

Usual cause

Heater element open, so refrigerant migrates to the compressor shell during the off cycle.

What the repair involves

Heater continuity is checked with an ohmmeter; an open heater is replaced.

Sources: Carrier service manual 24-25-2SM · Daikin / Goodman service manual RS6200006

What you see

Compressor shuts off on a pressure trip, often on hot days; unit may lock out.

Usual cause

Most often the switch is doing its job: high pressure from a dirty or blocked outdoor coil or a stopped fan, or low pressure from loss of charge, low indoor airflow or dirty filters. Less often the switch itself does not open and close at its rated pressures.

What the repair involves

The cause of the abnormal pressure is found and corrected first. A switch that does not operate at its rated pressures is replaced; removing one means recovering the system charge.

Checking a switch for continuity is electrical work; replacing one opens the refrigerant circuit.

Sources: U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · Carrier service manual 24-25-2SM

What you see

System shuts off in cooling until the drain is cleared, or water overflows and damages the area around the indoor unit.

Usual cause

Drain line or drain channels clogged, commonly with mold growth; where a condensate switch is installed it opens when water rises in the pan and shuts the outdoor unit off.

What the repair involves

The drain line is cleared and flushed, and any condensate pump is checked for debris. A condensate switch that stays open with a clear, dry pan is replaced.

Owner: clearing and flushing the drain line. Replacing a condensate switch is technician work.

Sources: U.S. DOE Energy Saver — Air Conditioner Maintenance (archived copy) · U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · ENERGY STAR — Maintenance Checklist · Daikin / Goodman service manual RS6200007

What you see

Weak airflow, reduced capacity, higher energy use; with enough restriction, ice forms on the indoor coil.

Usual cause

Filter clogged, so airflow drops and dirt bypasses onto the indoor coil; the coil also collects dirt over time even with a clean filter.

What the repair involves

The filter is cleaned or replaced on the schedule the manufacturer or installer gives. A dirty indoor coil is inspected and cleaned by a contractor.

Owner: the filter. Cleaning the indoor coil is contractor work per DOE.

Sources: U.S. DOE Energy Saver — Air Conditioner Maintenance (archived copy) · ENERGY STAR — Maintenance Checklist · Bosch service bulletin

What you see

Indoor blower does not run, or rocks without starting; little or no air at the registers while the outdoor unit runs.

Usual cause

No line voltage or no 24 V signal at the motor (open fuse or breaker, miswired or unseated harness), a failed motor or control module, or moisture in the motor or module.

What the repair involves

Line voltage and the 24 V signal are checked at the motor connectors before the motor is condemned; if both are present and the motor still will not run, the motor or its control module is replaced. A rough-feeling ECM shaft is normal.

Sources: ENERGY STAR — Maintenance Checklist · Daikin / Goodman service manual RS6200006

What you see

Gradual loss of cooling and heating capacity; system found low on refrigerant with the leak traced to the indoor coil.

Usual cause

Corrosion of copper tubing. In formicary corrosion, airborne pollutants from building materials and household products attack the copper. Manufacturers also list incorrect installation, inadequate maintenance and freezing.

What the repair involves

The leak is located and confirmed; a leaking indoor coil is normally replaced, the repair is tested, and the system is evacuated and charged to specification.

Sources: U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · Bosch service bulletin

What you see

Poor capacity with pressures and superheat that do not respond when the valve's sensing bulb is cooled or warmed.

Usual cause

Valve restricted, power element faulty, or equalizer tube plugged. NIST also lists a malfunctioning TXV among installation errors found in monitored heat pumps.

What the repair involves

The valve is tested by cooling and warming its bulb. A valve that does not respond is replaced together with the filter drier, after the charge is recovered; the system is then evacuated and recharged.

Sources: NIST Technical Note 1848 — Installation Faults on Heat Pump Performance · Daikin / Goodman service manual RS6200006

What you see

Auxiliary heat does not come on, or only part of it does, when the heat pump cannot hold temperature or during defrost.

Usual cause

Heater element open, limit control open, or one-time fuse link open. Limits and fuse links exist to protect against overheating from low airflow, so an open one points to an airflow problem to be found.

What the repair involves

Each element, limit and fuse link is tested for continuity and the open part is replaced. An open safety is never wired around; the reason it opened is determined.

The manual describes how the heat sequencer stages the elements but gives no sequencer failure test; sequencer failure itself is not documented in the sources opened.

Sources: Daikin / Goodman service manual RS6200006

What you see

Rooms that do not reach temperature, long run times, high energy use; charge readings that do not make sense.

Usual cause

Leaky ducts and low airflow from faulty installation: undersized return ducts and grilles, wrong fan speed setting, fouled filters, long or pinched duct runs.

What the repair involves

Duct leakage is diagnosed and sealed; airflow is measured and corrected (fan speed, returns, filters) before the refrigerant charge is judged.

Sources: NIST Technical Note 1848 — Installation Faults on Heat Pump Performance · U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · U.S. DOE Energy Saver — Operating and Maintaining Your Heat Pump (archived copy)

What you see

Reduced capacity and efficiency and long run cycles; the indoor coil can ice; the low-pressure or loss-of-charge switch can trip.

Usual cause

Undercharging at installation, or a leak. DOE lists both; a low charge does not by itself prove a leak.

What the repair involves

The system is checked for leaks; any leak is repaired and the repair tested before refrigerant is added, then the charge is set to the manufacturer's specification by measurement.

Sources: U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · NIST Technical Note 1848 — Installation Faults on Heat Pump Performance · EPA — Section 608 Technician Certification

What you see

Looks like a low charge (low suction and discharge pressures) but subcooling is normal to high; a temperature drop, or frost in severe cases, at the point of restriction.

Usual cause

A restricted component in the liquid line, such as a blocked filter drier.

What the repair involves

The restriction is located by the temperature drop, the restricted part and the drier are replaced, and the system is evacuated and recharged.

Sources: Daikin / Goodman service manual RS6200006

What you see

Excessively high head pressure; higher power draw and lower efficiency.

Usual cause

Air or nitrogen left in the system by incomplete evacuation at installation or after a repair; or too much refrigerant added. NIST warns that the first can be mistaken for the second.

What the repair involves

With the system off and equalized, pressure is compared with the temperature of the coldest coil to tell the two apart. Non-condensables are removed by recovering the charge and evacuating; an overcharge is corrected by recovering the excess.

Sources: NIST Technical Note 1848 — Installation Faults on Heat Pump Performance · Daikin / Goodman service manual RS6200006

What you see

No call reaches the outdoor unit (no 24 V at the control terminals), wrong staging, or heating and cooling operating against each other; room temperature read incorrectly.

Usual cause

Fault in the thermostat or its wiring, a thermostat that is not a heat pump model, incorrect control wiring, or a thermostat exposed to direct sunlight or obstructed.

What the repair involves

Control voltage is checked at the unit and the thermostat and wiring are checked for continuity, then repaired or replaced. The control sequence is verified so heating is locked out on a cooling call and the reverse.

Sources: NIST Technical Note 1848 — Installation Faults on Heat Pump Performance · U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · U.S. DOE Energy Saver — Operating and Maintaining Your Heat Pump (archived copy) · Daikin / Goodman service manual RS6200006

What you see

Nothing responds to the thermostat; no 24 V between R and C at the indoor unit; a diagnostic module flags low control voltage.

Usual cause

Failed transformer, bad wiring or splices on the transformer circuit, an overloaded control circuit, or an open low-voltage fuse.

What the repair involves

Secondary and primary voltage are measured; with line voltage present at the primary and good wiring, a transformer with no output is replaced. The reason for an open fuse or overload is found before the fuse is replaced.

Sources: Daikin / Goodman service manual RS6200006

What you see

System, or just the outdoor unit, is completely dead.

Usual cause

Breaker or fuse opened once as a nuisance, or opened because of a real electrical fault; disconnect open.

What the repair involves

After the unit has been off for about five minutes, the breaker may be reset one time. If it trips again it is left off and the electrical fault is found and corrected.

Owner: one breaker reset, once. A second trip, a blown fuse or anything inside the disconnect is technician work.

Sources: U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · Daikin / Goodman service manual RS6200006

What you see

Control board shows a network or communication error code and equipment does not run or runs in a limited mode.

Usual cause

No power to one of the units, an open fuse, or a fault on the data lines between indoor unit, outdoor unit and thermostat.

What the repair involves

Power and fuses are checked, then the voltages on the two data lines are measured to decide whether the network wiring or a control board is at fault; the faulty wiring or board is corrected or replaced.

Sources: Daikin / Goodman service manual RS6200007

What you see

Unit stops and displays an inverter-module protection or drive-error code.

Usual cause

The inverter module's current, voltage or temperature protection has tripped. The manuals list wiring between board and compressor, the IPM itself, the outdoor fan assembly, the compressor and the outdoor board as the parts to check.

What the repair involves

Wiring to the compressor, the IPM, the outdoor fan and the compressor are checked in the order the flowchart gives; the part that fails its check is replaced, most often the IPM or outdoor board. A failed compressor is refrigerant work.

Sources: Carrier ductless service guide 38MARB · Bosch ductless installation and service manual

What you see

Sensor error code on the display. With one sensor failed the unit keeps running; with more than one it stops.

Usual cause

Sensor open or short circuited, a loose connection between sensor and board, or a faulty board.

What the repair involves

The connection is checked, then the sensor is unplugged and its resistance compared with the manufacturer's temperature-resistance table. A sensor that is off the table is replaced; if the sensor is good the board is suspect.

Sources: Carrier ductless service guide 38MARB · Bosch ductless installation and service manual

What you see

On the units cited, a shorted EEV coil shows up as an indoor/outdoor communication error with the board's low-voltage DC supply pulled down.

Usual cause

Short circuit in the EEV coil loading the outdoor board's 5 V / 12 V supply.

What the repair involves

The EEV is unplugged; if the board's DC supply and power LED recover, the EEV coil is replaced. Only the coil is changed, so the refrigerant circuit stays closed.

Narrow support: both manuals mention the EEV coil only inside the communication-error flowchart. Other EEV faults (valve stuck open or closed) are not documented in the sources opened; replacing the valve body would be EPA 608 work.

Sources: Bosch ductless installation and service manual · Carrier ductless service guide 38MARB

What you see

System found low on refrigerant with the leak at a flare connection on the line set.

Usual cause

Flare assembled too loose to seal or tightened until it cracks or splits; damaged or reused flares; aftermarket flare nuts.

What the repair involves

A leaking flare is not tightened further. It is cut out and a new flare is made, torqued to the manufacturer's value with a torque wrench; the line set is then pressure tested with nitrogen, evacuated, and the charge restored.

No federal or manufacturer source opened says how often flares leak. The sources below are installation best-practice guides (a New York State agency and a regional non-profit).

Sources: NYSERDA — installation guidance · NEEP — Guide to Installing Air-Source Heat Pumps in Cold Climates

What you see

A water-level alarm code and the unit stops; or water around the indoor unit.

Usual cause

Water-level switch faulty or not seated, condensate pump faulty, wiring mistake, or a faulty indoor board; a blocked drain pan or drain line.

What the repair involves

The drain pan and line are cleared. The water-level switch is checked and reseated or replaced; if the alarm persists the pump is replaced.

Sources: U.S. DOE Energy Saver — Common Air Conditioner Problems (archived copy) · Bosch ductless installation and service manual · GREE — How to Deep Clean Moldy Ductless Mini-Split Units

What you see

Musty odor, visible residue, reduced airflow and efficiency.

Usual cause

Dirt, moisture and organic matter build up on the blower wheel and grow mold, especially in humid conditions.

What the repair involves

With power off and locked out, the covers and louvers are removed and the blower wheel, louvers and filters are cleaned with an approved coil/disinfectant cleaner, rinsed at low pressure and dried; the drain pan and line are confirmed clear.

Source is a manufacturer's website article addressed to contractors, not formal service literature.

Sources: GREE — How to Deep Clean Moldy Ductless Mini-Split Units

What you see

An indoor/outdoor communication error code; the system will not run.

Usual cause

Incorrect installation of the indoor-to-outdoor control wire (the wiring is polarity sensitive), electromagnetic interference, or a faulty indoor or outdoor board.

What the repair involves

Power is cycled, then the wiring between the units is checked and corrected or replaced; in a high-EMF location a shielded cable is used. If wiring is good the boards are tested and the faulty one replaced.

Sources: Carrier ductless service guide 38MARB · Bosch ductless installation and service manual

What you see

A fan-speed error code, indoor or outdoor, and the unit turns off.

Usual cause

Fan speed stays too slow or too fast: connection wires, the fan assembly or motor, or the board driving it.

What the repair involves

Power is cycled, then wiring, the fan assembly and motor and the board output are checked; the failed part is replaced.

Sources: Carrier ductless service guide 38MARB