What Are Aircon Contactors and Relays, and Why the Clicking?
Electrical switches inside the outdoor unit that complete the power circuit to the compressor and fan motor. When the contacts wear out, the outdoor unit clicks but never starts — symptoms easily mistaken for a weak capacitor or control board fault.
What the contactor and relay does
A contactor is an electrical switch inside the outdoor unit that turns on power to the compressor and fan motor. When the indoor unit sends a start signal, the contactor closes to complete the power circuit and let current flow. A relay is a smaller switch that handles lower-power control signals between components. Together, the contactor and relay form the switching path that connects your indoor commands to outdoor action — when either one fails, the outdoor unit stays silent or responds inconsistently because the power path is broken at the switch.
| Category | Electrical |
|---|---|
| Typical replacement cost | Varies |
| Replacement timeline | Same-day |
- Category
- Electrical
- Typical replacement cost
- Varies
- Replacement timeline
- Same-day
Contactor and relay failure signs
What you observe, what causes it, and how a technician confirms or rules out each path.
| What you observe | Likely causes | How we verify |
|---|---|---|
| Indoor fan runs but outdoor unit is silent | Worn contactor contacts failing to close fully, Pitted or corroded contact surfaces from repeated arcing | Verify the control board sends the start signal, then check whether the contactor closes — no closure with valid signal confirms a worn switch. |
| Clicking sound from outdoor unit but no cooling | Contactor attempting to close but contacts not making solid connection, Weak capacitor preventing compressor from starting after contactor closes | Test the capacitor first since a weak capacitor mimics this exactly; if capacitor is healthy, inspect contactor surfaces. |
| Outdoor unit works some days but not others | Intermittent contactor contact closure, Contact surfaces marginal and sensitive to position | Observe multiple startup attempts and inspect contacts for visible burn marks or pitting. |
- What you observe
- Indoor fan runs but outdoor unit is silent
- Likely causes
- Worn contactor contacts failing to close fully, Pitted or corroded contact surfaces from repeated arcing
- How we verify
- Verify the control board sends the start signal, then check whether the contactor closes — no closure with valid signal confirms a worn switch.
- What you observe
- Clicking sound from outdoor unit but no cooling
- Likely causes
- Contactor attempting to close but contacts not making solid connection, Weak capacitor preventing compressor from starting after contactor closes
- How we verify
- Test the capacitor first since a weak capacitor mimics this exactly; if capacitor is healthy, inspect contactor surfaces.
- What you observe
- Outdoor unit works some days but not others
- Likely causes
- Intermittent contactor contact closure, Contact surfaces marginal and sensitive to position
- How we verify
- Observe multiple startup attempts and inspect contacts for visible burn marks or pitting.
How we verify a contactor and relay fault
Diagnostic steps in order. Cheaper, more common causes get ruled out first so you do not pay for the wrong fix.
Test the capacitor first — a weak capacitor is more common and mimics contactor failure closely.
Tools: Capacitor tester, Multimeter
Healthy reading: Capacitor holds rated capacitance under test.
Check whether the control board is sending the start signal to the contactor.
Tools: Multimeter
Healthy reading: Control voltage appears at the contactor coil on cooling demand.
Observe whether the contactor closes and completes the power circuit when the signal arrives.
Tools: Multimeter
Healthy reading: Contactor closes audibly and continuity passes through the contacts.
Inspect contact surfaces for visible burn marks or pitting that confirm mechanical wear.
Healthy reading: Contact surfaces are clean and unpitted.
Replacing the contactor and relay
When replacement is the right call, when monitoring is fine, and when delay creates real risk.
Replace
Replace the contactor only after testing confirms the switch itself is broken and the capacitor has been ruled out. Contactor replacement is a low-cost repair — the real value lies in testing correctly to avoid swapping it when the capacitor or control board is the actual problem.
You can wait
If the unit starts most of the time but occasionally fails on the first attempt, the contactor may still be functional enough for now. An intermittent contactor tends to fail completely soon.
Do not wait
If the outdoor unit fails to start on most attempts. Repeated failed starts put mechanical stress on the compressor, and early diagnosis avoids being stuck without cooling on a hot day.
If you proceed
Contactor replacement is a quick, straightforward repair once the fault is confirmed through testing. The part is common and usually available without delay.
Before approving any replacement, ask what specific test result confirmed the contactor as the fault. Proper diagnosis at this stage prevents paying for a contactor when the capacitor or control board is the real cause.
Frequently asked questions about the aircon contactor and relay
What are the warning signs of a failing contactor and relay?
When should I switch off the aircon because of the contactor and relay?
How does a technician confirm the contactor and relay has failed?
Does the contactor and relay need replacing, or can it be repaired?
What is the contactor and relay responsible for?
Describe your situation
Outdoor unit not starting? Describe what you see on WhatsApp.