Aircon won't cool below 24°C on the Sunshine Coast - the 6-point fault tree.

The 6-point fault tree we walk in the order we walk it

When a Sunshine Coast aircon runs but cannot pull room temperature below about 24 degrees, the cause is almost always one of these six, in approximate frequency order. We work the tree from easiest to hardest because that is the cheapest path to the answer for you.

1. Dirty filter and evaporator coil (~30% of "won't cool" calls)

Airflow over the indoor evaporator coil is half the cooling equation. Block 30% of the airflow with a fouled filter and a slimed coil - which happens fast in humid subtropical conditions - and you lose roughly 30% of the cooling. The unit runs longer, draws more power, and the room never quite gets there. Diagnostic tell: indoor return-air to supply-air temperature differential is below 8 degrees (should be 11-14 degrees on a healthy system). Fix: chemical coil clean and filter wash, about $140 to $180 add-on to the diagnosis. Often resolves the call entirely.

2. Blocked outdoor condenser (~20%)

The outdoor unit needs to reject heat to ambient, and on the Sunshine Coast we get coast-grass clippings, paperbark leaves, frangipani fall, and on coastal jobs salt-encrusted lint that mats the condenser fins. A 50% blocked condenser drives discharge pressure up by 20-30%, which raises compressor amp draw, drops system capacity, and on hot days triggers the high-pressure cutout. Diagnostic tell: discharge pressure on the gauges runs 20%+ above expected. Fix: fresh-water back-rinse from inside out, about $80 to $120 add-on. On salt-zone jobs we add the polymer inhibitor too.

3. Low refrigerant charge (~25%)

The great impersonator. Low charge looks like every other fault - it short-cycles, it ices up, it labours. The fingerprints on the gauges are clean though: suction pressure low (below 900 kPa on a running R32 split at 28 degrees ambient is suspect), suction superheat high (above 12 degrees), discharge-line temperature high (above 95 degrees), subcooling low (below 6 degrees). If we see that pattern we move to leak detection - see our refrigerant leak detection page for the two-stage process. We do not just top up; that is illegal under our ARCtick licence and is throwing your money away.

4. Failing capacitor (~10%)

The outdoor unit has a run capacitor that holds the start torque for the compressor and condenser fan. Capacitors are a wear part - they degrade slowly over years and faster in coastal heat. A capacitor at 30 microfarads instead of its rated 45 microfarads makes the compressor labour, run hot, and underperform without throwing a fault code. Diagnostic tell: microfarad reading on the capacitor with our meter is below 90% of nameplate. Fix: $90 to $160 supplied and fitted, takes about 15 minutes once we are on site.

5. Undersized for the room / heat load (~10%)

A 2.5 kW split is rated for around 25 square metres of well-insulated bedroom. Sell that same unit into a 45 square metre open-plan living area facing west in Buderim through 36-degree February afternoons, and it will run flat out without getting below 24 degrees because the room is rejecting heat faster than the unit can remove it. Diagnostic tell: gauge readings are all fine and the unit is running at 100% output. Fix: there isn't one short of upsizing the unit. We do an honest heat-load calculation (square metres, glazing, orientation, insulation) and tell you what kilowatt rating the room actually needs.

6. Compressor end-of-life (~5%)

The death sentence. After 10 to 15 years (less in coastal salt zones with no servicing, more on well-maintained inland systems) the compressor's internal valves wear and it stops building head pressure. Diagnostic tell: compression ratio below 2.0, discharge-line temperature low (the compressor is barely doing anything), amp draw collapsed. On any reasonable cost-benefit a compressor replacement on an aged unit is uneconomic - labour plus refrigerant plus a 12-month parts warranty on a system whose other components are also tired pushes the math toward replacement. We say so plainly when that is the answer.

The honest diagnosis call-out

Flat fee $130 to $180 to come out, measure all the things, give you a written diagnosis and a quote for whatever the actual fix is. The diagnosis fee is rolled into the repair if you proceed within 30 days. If we tell you the unit is dead, we tell you that - no upsell, no replacement quote unless you ask for one. About one in five "won't cool" calls turns out to be a 20-minute filter clean, and we will happily do that and bill an hour rather than waste your time.

What healthy gauge readings look like on an R32 split

For reference, on a 5 kW R32 split running in cooling at 28 degrees ambient, 22 degrees return air, indoor at design load: suction pressure 950-1050 kPa, discharge pressure 2700-3000 kPa, suction superheat 5-10 degrees C, subcooling 8-12 degrees C, discharge-line temperature 60-80 degrees C, compressor amp draw 5-7 amps, indoor return-to-supply differential 11-14 degrees. R410A systems run higher pressures (suction 1100-1200 kPa, discharge 2900-3200 kPa) for similar load. We write these numbers on every diagnosis sheet so you can see what we measured and where it sat relative to spec.

What to do right now before we get there

Two quick things you can do that may save the call-out entirely:

Pull the front cover off the indoor unit and wash the two foam filters in the laundry sink with mild detergent. Air-dry them, refit. If the filter was visibly grey-black with dust, that may have been the whole problem. Re-test the cooling for an hour and see if it pulls below 24 degrees.

Walk outside and check the outdoor condenser is not blocked. Pull obvious leaf litter and grass clippings off the front grille. If the fins are matted with salt or dust, a garden hose on a gentle setting from the inside of the unit out through the fins (with the power off at the isolator) can shift it. If after both of those it still will not cool, give us a call - that rules out two of the six causes and we can focus the diagnosis on the others.

Service area & booking

Fault diagnosis and repair on aircon systems across Maroochydore, Mooloolaba, Buderim, Caloundra, Kawana, Sippy Downs, Nambour, Noosa, Coolum and the hinterland. ARCtick refrigerant-handling licence on every job, QBCC licensed contractor, fully insured. Book via the quote form or email quotes@sunshinecoastairconservice.com.au with your suburb, the brand and approximate age of the unit, and a description of the symptom (running constantly, ice on the suction line, weak air, no cool air at all).

FAQs about underperforming aircons on the Sunshine Coast

My aircon runs all day and won't get below 24°C - is it stuffed?

Probably not. Six common causes produce exactly that symptom and only one - compressor end-of-life - is fatal. The other five (dirty filter/coil, low refrigerant, blocked condenser, weak capacitor, undersized for the room) are all fixable. The right starting point is a measured diagnosis ($130-$180) - anyone quoting you a new system without first reading gauges on the old one is guessing.

What should the discharge-line temperature be?

On a healthy R32 split at 28°C ambient: 60-80°C at the discharge line. Above 95°C suggests low charge or a restriction. Below 45°C means the compressor isn't compressing properly - typically end-of-life. It's one of the cleanest single diagnostic readings for separating a tired compressor from a low-gas job.

How do I tell if the compressor is dead vs. just low on gas?

Static pressure with the system off should match saturation pressure for the refrigerant at ambient - if it does, gas is there. Running compression ratio (discharge ÷ suction) should be 2.5-3.5 under load; below 2.0 is a worn compressor. Amp draw: a 5 kW compressor should pull 5-7 A under load. We run all three before saying "replace".

What does low refrigerant masquerade as?

Everything. It looks like a tired old system, an iced-up coil, an airflow fault, or a compressor problem - all four. Untrained eyes call it "needs replacing" and sell a new unit when a leak repair plus regas would have saved thousands. Gauge readings tell the truth: suction superheat above 12°C and discharge-line temp above 95°C is a low-charge fingerprint. Find the leak, fix the leak, regas.