Aircon Contactor and Relay
This is the overview page for outdoor switching faults. A contactor or relay issue can stop startup, but we still need checks to confirm which switching point failed.
What This Part Does
The contactor is an electrical switch inside the outdoor unit. When the aircon calls for cooling, the outdoor PCB sends a signal to the contactor. The contactor closes and connects mains power to the compressor and outdoor fan motor.
The relay performs a similar switching function for lower-power circuits — controlling the outdoor fan or signalling between components. Both parts are high-cycle components. They open and close every time the aircon starts and stops.
When a contactor or relay fails, the signal to start a component arrives but the power never flows. The component looks dead even though nothing is wrong with it.
How You Would Notice
The indoor unit runs normally — fan blows, lights are on — but no cooling arrives. The outdoor unit is silent or the outdoor fan runs but the compressor does not start.
You may hear a click or buzz from inside the outdoor unit when the aircon calls for cooling. That is the contactor receiving the signal but failing to close the power circuit.
In some cases the outdoor unit starts intermittently — working on some days and not others. This is a contactor that is partially failed but still makes contact under some conditions.
- Indoor unit runs but no cooling — outdoor unit silent
- Click or buzz from outdoor unit but compressor does not start
- Intermittent outdoor unit operation with no apparent pattern
It Might Not Be The Contactor
A failed run capacitor creates an identical symptom — the compressor tries to start but cannot. The capacitor fails to provide the starting torque, the compressor hums and stops, and no cooling arrives. The capacitor is a cheaper and more common fault than the contactor.
An outdoor PCB fault can prevent the signal from ever reaching the contactor. If the board is not sending the command to close, the contactor cannot respond regardless of its condition.
A tripped thermal cutout inside the compressor also stops the outdoor unit. The compressor has its own protection. If it has overheated and tripped, it will not start until it cools down — and may have an underlying cause worth investigating.
How We Check
We start with the capacitor — it is the more common and cheaper cause of the same symptom. We test it under load and confirm whether it is holding its rated value.
If the capacitor tests fine, we test the contactor directly. We check whether the signal is arriving from the PCB, and whether the contactor is closing when the signal is present. A contactor that receives a signal but does not close is confirmed faulty.
We also check for signs of contact burn or pitting on the contactor's contact faces. Heavy pitting means the contacts have been arcing — often from a contactor that was failing intermittently for some time.
If the contactor is fine and the signal is not arriving, we trace back to the outdoor PCB.
What We Find And What Happens Next
Most outdoor-unit-not-starting cases trace to the capacitor or contactor. Both are accessible, testable, and replaceable without touching the compressor.
| Finding | Next Step |
|---|---|
| Capacitor out of range | Replace capacitor, retest compressor start |
| Contactor not closing under signal | Replace contactor, retest outdoor unit |
| Signal absent — PCB not sending command | Outdoor PCB assessment |
| Contactor fine, signal present, compressor silent | Compressor assessment |
About The Repair
Contactor replacement is a straightforward electrical job inside the outdoor unit. The part is accessible without major disassembly. Sourcing is generally reliable for common brands and capacity ranges.
We match the replacement contactor to the unit's voltage and current rating. An undersized contactor will fail again quickly under the load it was not rated for.
If a contactor has burned or pitted contacts, we also check what caused the excessive arcing. A contactor that failed because of a slow-starting compressor may indicate a compressor issue worth noting.
After Replacement
After contactor replacement, we confirm the outdoor unit starts cleanly — compressor and fan motor both energise on the first call for cooling. We listen for any abnormal startup sounds and check that both components reach normal running speed.
We run the system through a full cooling cycle and confirm cooling is restored to the indoor unit. Electrical resistance checks on the new contactor confirm it is seated and wired correctly.
If the unit was starting intermittently, we note the pattern in the service record. Intermittent electrical faults are hard to confirm on a single visit. Documenting what was found keeps the history clear if a related issue appears later.
When We Tell You To Wait
If the outdoor unit is starting intermittently and cooling arrives on most cycles, the contactor is degrading but not yet fully failed. Monitoring is reasonable if the situation is not urgent. Intermittent contactors tend to fail completely rather than recover — replacing it early avoids a breakdown on a hot day.
If the unit is not starting at all and the indoor fan is the only thing running, this is not a monitoring situation. No cooling means the compressor is not running, and the fault needs to be identified.
We will tell you on-site whether the fault is confirmed, intermittent, or unclear. If it is unclear, we will give you the specific condition to watch for before calling us back.