This is one of the most common emergency calls we get across Perth during summer. The good news: in many cases, the fix is simple and takes five minutes. In others, it needs a professional — but knowing which is which saves you time and stops you from making things worse.
Work through this list in order. Start at the top.
Is It Actually Not Running — or Just Quiet?
Short answer: Check for fan movement and sound before assuming it’s dead.
Some Perth outdoor units — particularly newer inverter models — run very quietly at low capacity. Before doing anything else:
- Go outside and look at the fan on top or side of the outdoor unit
- If the fan is spinning slowly and there’s a faint hum — the unit is running, just at low speed
- If the fan is completely still and there’s no sound at all — it’s not running
If it’s genuinely silent and still, move to the checks below.
Check 1 — Thermostat Settings
Short answer: Wrong mode or wrong temperature setting stops the outdoor unit from starting.
| What to Check | What It Should Say |
| Mode | COOL — not FAN or HEAT |
| Set temperature | At least 3°C below current room temp |
| Controller batteries | Replace if display is dim or blank |
This sounds too simple to be the answer — but it accounts for a significant number of calls we attend, particularly at the start of summer when the system hasn’t been used in months and settings have drifted.
Check 2 — Circuit Breaker at the Switchboard
Short answer: A tripped breaker cuts power to the outdoor unit while the indoor unit keeps running.
The outdoor condenser has its own dedicated circuit breaker — separate from the indoor unit. If this breaker trips, the indoor unit continues operating normally while the outdoor unit goes silent. This is exactly the symptom most Perth homeowners describe.
What to do:
- Go to your main switchboard
- Find the breaker labelled “AC outdoor,” “aircon,” or “condenser”
- If it’s in the middle position (half-tripped) — switch it fully OFF then back ON
- Go outside and check if the outdoor unit starts
If the breaker trips again immediately — stop. Do not keep resetting it. A breaker that keeps tripping means something is drawing too much current. Call a licensed technician. Repeatedly resetting a tripping breaker can damage the compressor or create a fire hazard.
Check 3 — Isolation Switch Near the Outdoor Unit
Short answer: There’s usually a second switch right next to the outdoor unit that can be accidentally turned off.
Most Perth homes have an isolation switch — a small box or switch on the wall within a metre or two of the outdoor unit. This switch is sometimes turned off during lawn mowing, garden work, or a previous service visit and simply forgotten about.
Check that this switch is in the ON position. If it was off, turn it back on and test the system.
Check 4 — Debris Around and Inside the Outdoor Unit
Short answer: A blocked outdoor unit can overheat and shut itself off as a safety measure.
Perth’s summer winds bring leaves, seed pods, and fine debris that accumulate around and inside outdoor units. When airflow is sufficiently restricted, the unit’s thermal protection shuts it down automatically.
Check for:
- Leaves, grass clippings, or debris packed against the fins on the sides
- Anything sitting on top of the unit blocking the fan exhaust
- Vegetation that has grown up around the unit — there should be at least 50cm clearance on all sides
Remove any visible debris, allow the unit to cool for 20 to 30 minutes, then attempt to restart.
Experience — In Perth’s eastern suburbs — Midland, Armadale, Forrestfield — debris accumulation in and around outdoor units is faster than most homeowners expect, particularly after strong easterly wind events. A unit that runs fine on calm days but keeps shutting off on windy ones is almost always a debris and airflow issue.
Check 5 — The Indoor Unit’s Condensate Drain
Short answer: A blocked drain pan inside triggers a safety switch that shuts the whole system down — including the outdoor unit.
This one surprises most people. If the condensate drain pan inside the home overflows because the drain line is blocked, a float switch cuts power to the system to prevent water damage. The result looks exactly like a power failure — both units stop, or just the outdoor unit stops depending on the system.
Signs this might be the cause:
- Water dripping from the indoor unit or nearby ceiling
- The system worked fine until recently and then suddenly stopped
- You notice a gurgling sound from the indoor unit area before it stops
If you see water, do not restart the system — address the drain first or call for service.
Check 6 — Has the System Overheated?
Short answer: On extreme Perth days, the outdoor unit can shut itself off temporarily as a safety measure.
On days above 40°C — which Perth experiences regularly during summer heatwaves — the outdoor unit’s high-pressure safety switch can trip if the condenser coil is dirty or airflow is restricted. The unit shuts off to protect the compressor, then automatically resets once it cools down.
What to do:
- Switch the system off at the controller
- Wait 20 to 30 minutes for the unit to cool
- Restart and monitor
If it shuts off again after a short period on a very hot day, dirty condenser coils restricting heat rejection is the most likely cause. This requires professional cleaning.
Expertise — High-pressure safety switch trips are significantly more common in Perth than in cooler Australian cities because our sustained 40°C+ days create the extreme temperature differential that pushes systems to their limits. A system that copes fine on a 35°C day but trips out on a 42°C day is usually telling you the condenser coil needs cleaning, not that the system is failing.
When to Stop Checking and Call a Professional
Some causes of an outdoor unit not running are not safe or appropriate for homeowners to investigate. Call a licensed technician if:
| Situation | Why |
| Breaker keeps tripping after reset | Electrical fault — continuing risks compressor damage or fire |
| Burning smell from outdoor unit | Electrical component failure — switch off immediately |
| Loud humming but fan not spinning | Failed capacitor — needs professional replacement |
| Clicking sounds but unit won’t start | Failed contactor or capacitor |
| Unit was working then stopped mid-cycle with no obvious cause | Could be refrigerant, compressor, or electrical — needs diagnosis |
| Any damage to wiring or external components visible | Do not touch — call immediately |
Frequently Asked Questions About Outdoor AC Unit Not Running
1. Will running the indoor unit fix the problem while the outdoor unit is off?
No — and it can make things worse. Switch the system off until the outdoor unit issue is resolved.
2. How do I know if my outdoor AC unit is overheating or has actually failed?
An overheated unit restarts after 20 to 30 minutes. One that restarts briefly then stops again is cycling on a safety switch. One that won’t restart at all after cooling has likely experienced a component failure.
3. Is it safe to leave my outdoor unit off during a Perth heatwave while I wait for a technician?
Yes. Leaving it off is always safer than forcing repeated restarts on a faulty unit — which risks compressor damage, one of the most expensive AC repairs.
4. Could a recent power surge have caused my outdoor unit to stop?
Yes. After a Perth summer storm, check the breaker first. If it holds after reset and the unit runs normally — you’re likely fine. New sounds or repeated stopping after a storm means a technician should check the capacitor and contactor.
5. Why does my outdoor unit work in heating mode but not cooling mode?
This points to cooling-specific components — the high-pressure switch, capacitor under cooling load, or a refrigerant pressure issue. A useful clue to give a technician before they arrive.
To Wrap Up
An outdoor AC unit that’s gone silent during a Perth summer is almost always one of a short list of causes — a tripped breaker, wrong thermostat settings, debris blocking airflow, or an overheated unit that’s reset itself. Start with the checks above before assuming anything more serious.
The causes that need a professional — a failed capacitor, contactor, refrigerant issue, or compressor fault — have clear signs that separate them from the DIY-fixable ones. Knowing which situation you’re in before you call means faster service and a clearer conversation with whoever attends.