QualifiedMedium risk

Air-con runs but isn't cooling (electrical checks)

The unit runs (fan blows) but doesn't cool — from an electrical standpoint, checking whether the compressor/outdoor unit is actually being commanded and powered before handing to refrigeration.

Start fault-finding

Safety first

Isolate before electrical checks. Cooling performance often involves refrigerant — that's refrigeration-tech territory; here we confirm the electrical side.

Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

  1. 1

    Outdoor unit/compressor not running

    Most likely

    The condenser/compressor isn't running, so no cooling — could be electrical (supply/contactor/cap) or refrigeration.

  2. 2

    Mode/setpoint wrong on the controller

    #2

    Set to fan-only, a too-high setpoint, or a schedule limiting it.

  3. 3

    Outdoor fan/contactor/capacitor fault

    #3

    A failed run capacitor, contactor, or outdoor fan motor stops the outdoor unit working.

  4. 4

    Refrigerant/mechanical issue (hand off)

    Least likely

    Low refrigerant or a mechanical fault — refrigeration tech required.

Reports are saved on this device to reflect what you actually find.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 2
1

Check the controller mode/setpoint/schedule is actually calling for cooling.

Expected reading

Cooling mode, setpoint below room temp, not schedule-limited.

If it passes

It's calling for cooling — check the outdoor unit electrically.

If it fails

Wrong mode/setpoint/schedule — correct it.

Often leads toSplit-system air-con not running at allAir-con tripping the power
View all expected readings at once
1. Check the controller mode/setpoint/schedule is actually calling for cooling.
Cooling mode, setpoint below room temp, not schedule-limited.
2. Isolate and check the outdoor unit's supply, contactor, run capacitor, and fan when cooling is called.
Outdoor unit powered, contactor pulling in, fan running, capacitor healthy.

Fault-finding flowchart

The same logic as a decision tree.

  1. 1
    start

    Aircon not cooling

    → step 2
  2. 2
    decision

    Is the controller actually calling for cooling?

    Yes→ step 3No→ step 4
  3. 3
    decision

    Is the outdoor unit powered, commanded, and electrically healthy?

    Yes→ step 5No→ step 6
  4. 4
    result

    Correct mode/setpoint/schedule.

  5. 5
    result

    Electrically OK but not cooling — refrigeration technician.

  6. 6
    result

    Electrical fault (contactor/cap/fan/supply) — rectify.

Common mistakes apprentices make

  • Not checking the controller is actually in cooling mode.
  • Assuming refrigerant when a run capacitor/contactor has failed.
  • Attempting refrigerant work without a licence.
  • Not isolating before checking the outdoor unit.

When to stop & escalate

Once the electrical side is confirmed (unit powered and commanded), cooling-performance/refrigerant issues are for a licensed refrigeration technician.

If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.

Related faults

HVAC & air-con

Split-system air-con not running at all

A split-system (head unit + outdoor condenser) is completely dead — no response from the remote/controller — pointing at supply, isolator, controller, or the indoor PCB.

4 causes3 test steps
QualifiedHigh risk
HVAC & air-con

Air-con tripping the power

An air-con unit trips its breaker or safety switch — on start, when the compressor kicks in, or randomly — pointing at the compressor, an earth fault, or the supply/protection.

5 causes3 test steps
AdvancedHigh risk
HVAC & air-con

Ducted air-con system has no power / won't start

A ducted system (roof/cupboard fan unit + zones/controller) is dead or won't start — pointing at the dedicated supply/isolator, the controller, zone motors, or the unit's PCB.

4 causes3 test steps
QualifiedHigh risk

Learn the theory

How the gear and circuits behind this fault actually work.

Principle

How HVAC control works (electrical view)

A thermostat/controller calls for heating or cooling; the electrical side energises the unit — refrigerant is separate.