Outdoor Unit Guide
Air Conditioner Compressor Not Starting
An air conditioner compressor not starting is one of the most important faults to understand. The compressor is the heart of the system, and without it there is no cooling or heating at all. This guide explains the signs, why it happens on modern inverter and older units, and the few checks that are safe for owners.
Recognise It
How Do You Know the Compressor Is Not Starting?
The compressor lives inside the outdoor unit, usually behind a side panel. You cannot see it, but you can often hear and feel whether it is working. When it runs, there is a steady hum or whir and a gentle vibration through the casing.
When it fails to start, the signs are fairly consistent. The indoor fan runs, but the air stays at room temperature. Outside, the fan may spin while the unit stays strangely quiet, or nothing moves at all. Sometimes there is a click, a hum or a buzz, followed by silence.
Silent Outdoor Unit
No fan, no hum. The outdoor unit may not be receiving power or a start signal.
Fan Runs, No Hum
The outdoor fan spins, but the compressor never kicks in. Air indoors stays warm in cooling mode.
Click, Hum, Then Stop
The compressor tries to start and fails. This often comes with an error code or flashing lamp.
If the outdoor unit appears completely dead, our existing article on an outdoor unit not running covers that wider symptom. This page focuses on the compressor itself.
Inverter or Fixed Speed
Why Does the Type of Compressor Matter?
Nearly all split systems sold in Australia today use inverter compressors. An inverter board in the outdoor unit converts power and drives the compressor at variable speed. In contrast, older fixed speed units start the compressor directly with a contactor and a start capacitor.
The difference changes what can go wrong. On a fixed speed unit, a failed capacitor or contactor is a classic cause. On an inverter unit, the board, its power module, sensors or communication are more common suspects. The compressor itself can fail on either type, but it is not always the first thing to blame.
Inverter Systems
The board checks sensors, pressures and communication before allowing a start. If anything looks wrong, it holds the compressor off and usually shows a code. This is protection, not necessarily a dead compressor.
Fixed Speed Systems
These rely on simpler parts. A weak start capacitor may let the compressor hum without turning. A burnt contactor may stop power reaching it at all.
Ranked Causes
What Stops an Air Conditioner Compressor From Starting?
The list below runs from the simplest causes to the most serious. Many of the early ones are not compressor faults at all, which is good news.
Simpler Causes
- Mode set to fan only, dry or auto with no demand
- Set temperature already reached, so no call for cooling
- Built in restart delay after power cycling or mode changes
- Outdoor isolator switched off or breaker tripped
- Severe heat or dirt on the outdoor coil triggering protection
- Defrost cycle in progress during cold weather heating
Serious Causes
- Communication fault between indoor and outdoor units
- Faulty sensor making the board refuse to start
- Failed start capacitor or contactor on older units
- Inverter board or power module failure
- Low refrigerant triggering low pressure protection
- Seized compressor or failed motor windings
Note that a seized compressor is the least common cause on this list, even though it is often the first fear. Most no start faults are found elsewhere.
Safe Checks
What Can an Owner Safely Check?
The outdoor unit contains high voltage parts and capacitors that can hold a charge after power is off. Keep to these external checks only, and never remove panels from the outdoor unit.
Check the Mode and Set Point
Confirm the remote shows cool or heat, and that the set temperature is well away from the current room temperature. Replace the remote batteries if the display is faint.Allow the Restart Delay
After any change, wait at least five minutes. Most systems hold the compressor off for a short time to protect it.Check the Breaker and Isolator
Make sure the air conditioner circuit breaker is on. Then check the weatherproof isolator beside the outdoor unit is in the on position, without opening it.Power Cycle Once
Switch off at the remote, turn off the breaker for a few minutes, then restore power and try again.Clear the Outdoor Unit
Remove leaves, plants and anything stored against the unit. Leave clear space on all sides.Clean the Indoor Filters
With power isolated, clean the filters. Heavy blockage can trigger protection on some systems.Record Codes and Sounds
Write down any code or flashing pattern. Note whether you hear a click, hum or buzz outside when the system tries to start.
If the breaker trips when the compressor tries to start, stop resetting it. Our guide on an air conditioner tripping circuit breaker explains why. For general safe resets, see resetting common air conditioning faults yourself.
Brand Behaviour
How Do Major Brands Signal a Compressor That Will Not Start?
Inverter systems from every major brand monitor the compressor and its drive closely. Each manufacturer has its own code set, so the same fault reads differently on each. Below is a general overview, with links to code pages that exist on our site.
Daikin
Daikin outdoor units may show Daikin E5 for compressor overload or lock, Daikin L5 for inverter overcurrent and Daikin H0 for a compressor sensor system fault. Daikin U4 covers communication loss, which also stops the compressor.
Mitsubishi Electric and MHI
On Mitsubishi Electric systems, communication codes such as Mitsubishi E6 and Mitsubishi E9 stop the compressor, as can outdoor sensor faults like Mitsubishi U4. Mitsubishi Heavy Industries uses a separate code set, so always confirm the brand.
Panasonic
Panasonic may hold the compressor off with Panasonic H11 for communication loss or Panasonic H15 for a compressor temperature sensor fault. Panasonic F97 records compressor overheating.
Samsung
Samsung units commonly show Samsung E101 or Samsung E202 for communication loss. Samsung E416 relates to high discharge temperature, another reason the compressor may stop.
Fujitsu
Fujitsu uses Fujitsu 11 for serial communication faults, Fujitsu 63 for inverter errors and Fujitsu 94 for trip detection. A discharge sensor problem may appear as Fujitsu 71.
Actron ducted systems behave in a similar way. Communication codes such as Actron E5 and protection codes such as Actron E9 can stop the compressor. For a broader overview, visit our air conditioner error codes guide.
Protection Logic
Is the Compressor Broken or Just Being Protected?
This is the key question, and it often has a reassuring answer. Modern systems are designed to refuse to start the compressor whenever conditions look unsafe. That protects the most valuable part of the system from damage.
For example, if a temperature sensor reads nonsense, the board cannot judge whether the compressor is overheating. As a result, it keeps the compressor off. Replacing a faulty sensor in that case restores normal operation, and the compressor was never at fault.
Similarly, if the indoor and outdoor units cannot communicate, the outdoor unit has no instructions and will not start. The fault may be a loose wire, water damage in a terminal block or a failed board. Again, the compressor itself may be perfectly healthy.
Open more detail
Low refrigerant is another protective trigger. Many systems watch pressure or temperature patterns and stop the compressor if they suggest the charge is low. Running a compressor without enough refrigerant can overheat it, because the gas also carries heat away from the motor. In this case, the real job is to find and repair the leak before recharging.
When the compressor really has failed, the technician will usually find evidence such as shorted or open windings, a winding shorted to earth or a mechanical seizure. Those tests require proper instruments. A compressor replacement is a significant job involving refrigerant recovery, brazing, pressure testing and recharging by an ARCtick licensed technician. On older systems, the technician may discuss whether replacing the whole system makes more sense.
Australian Conditions
Does Weather Affect Whether the Compressor Starts?
Yes, quite strongly. Compressors work hardest at the extremes, and Australian cities offer plenty of those.
Heatwaves
During extended heat in Adelaide and Perth, outdoor units run at full load for days. High pressure and high discharge temperature protection can stop the compressor, especially if the coil is dusty.
Storms and Power Surges
Summer storms in Brisbane and Sydney bring power blips and surges. Inverter boards can be damaged, and the system may refuse to start afterwards.
Cold Mornings
On cold winter mornings in Melbourne and Adelaide, heating systems pause for defrost. The compressor may stop briefly, which is normal behaviour.
Salt Air and Humidity
Coastal homes see corrosion on outdoor electrical terminals and coils. Over time this can cause poor connections or communication faults that stop the compressor.
If your system stopped starting right after a storm, tell us when you book. That detail points the technician towards surge damage or water entry before anything else.
Professional Diagnosis
What Does a Licensed Technician Test?
Diagnosing a no start fault involves both electrical and refrigeration checks. The technicians we send hold the licences their work requires, including ARCtick licensing where refrigerant is involved.
Fault History
They read the current and stored codes from the indoor unit, controller or outdoor board.
Supply and Communication
Voltage at the outdoor unit and the signal between units are measured to rule out wiring problems.
Sensor Resistance
Temperature sensors are tested against the manufacturer’s values to find any faulty readings.
Board and Module Tests
On inverter units, the board and power module are checked for correct output to the compressor.
Capacitor and Contactor
On fixed speed units, the start components are tested, as they are common failure points.
Compressor Windings
Winding resistance and insulation to earth are measured to confirm whether the compressor itself has failed.
Safety
Is It Safe to Keep Trying to Start the System?
A couple of attempts after a proper power cycle are fine. Beyond that, repeated restarts are not a good idea. Each failed start can stress electrical parts, and a compressor trying to start against a fault can overheat.
Stop and book an inspection if the compressor clicks or hums without starting, if the breaker trips, or if you smell burning near the outdoor unit. In addition, do not try to tap or hit the compressor, and never open the outdoor unit. Capacitors can deliver a dangerous shock even after power is switched off.
- Fine: waiting through a short restart delay or defrost cycle
- Caution: a single power cycle, then test once more
- Stop: repeated clicks, humming or buzzing without starting
- Stop urgently: breaker trips, burning smell or smoke
If your system starts and runs but keeps cutting out, see our article on an air conditioner that keeps turning off. For warm air with the outdoor unit running, read air conditioner blowing warm air on cool mode.
Before You Call
What Information Helps the Technician Most?
A compressor fault can have many causes, so good information saves time on the day. A few minutes of note taking before you book can shorten the diagnosis.
Brand and Model
Find the model number on the side of the indoor unit or the rating plate on the outdoor unit. A photo of the label is ideal.
Codes and Lamps
Write down any code on the display or controller. For units with flashing lamps, count the flashes and note which lamp is blinking.
Sounds Outside
Note whether you hear a click, a hum, a buzz or nothing at all from the outdoor unit when the system tries to start.
Recent Events
Mention any storm, power cut, renovation, roof work or pest activity in the days before the fault appeared.
It also helps to know the rough age of the system and when it was last serviced. Older systems on their original parts have different likely faults from a unit installed only recently. If the system is still under the manufacturer’s warranty, have your purchase details handy, since warranty claims usually go through the installer or manufacturer.
Prevention
How Can You Help Your Compressor Last Longer?
Compressors are built to run for many years, but they rely on the rest of the system staying healthy. Simple care reduces the strain on them.
- Keep indoor filters and the outdoor coil clean
- Leave clear space around the outdoor unit for airflow
- Avoid switching the system on and off repeatedly in short bursts
- Have suspected refrigerant leaks repaired promptly
- Book a periodic service that checks electrical connections and running current
A compressor that runs with low refrigerant or a clogged coil works hotter and harder. Catching those problems early is the best protection for the most expensive part of your system.
Questions
Frequently Asked Questions
Does a Compressor Not Starting Mean It Is Dead?
Not usually. Communication faults, sensors, boards, capacitors and protection trips are all more common causes than a failed compressor.
Why Does the Outdoor Fan Run but Not the Compressor?
The board may be holding the compressor off for protection, or a component in the drive circuit may have failed. A technician needs to test it.
How Long Is the Normal Restart Delay?
It varies by brand and model, but a few minutes is typical. Waiting around five minutes before judging is sensible.
Can a Power Surge Stop the Compressor From Starting?
Yes. Surges can damage inverter boards and power modules. Mention any recent storm when you book.
Is a Humming Outdoor Unit a Bad Sign?
A steady hum while running is normal. A hum or buzz with no cooling, especially followed by a click, suggests the compressor is struggling to start.
Can Low Refrigerant Stop the Compressor?
Yes. Many systems stop the compressor to protect it when pressure or temperature patterns suggest a low charge. The leak must be repaired before recharging.
Is It Worth Replacing a Compressor?
It depends on the system’s age and condition. A technician can explain the options, including whether replacing the whole system is better value.
Book a Diagnosis
Where Can You Get a Compressor Fault Diagnosed?
Rapid Air Cooling books licensed technicians for every major brand in five capital cities. Choose your city below, or learn more about our air conditioning service Australia coverage.
Compressor Still Not Starting?
Note any code and the sounds you hear, then call us. We will arrange a licensed technician to find out whether it is protection, a part or the compressor itself.
Reviewed by AC repair and service specialist on 22 October 2026. How we write our articles