Your AC is running but not cooling: what is actually wrong
Nine causes, ordered from the ones you can check yourself to the ones that need a technician, with what each typically costs to put right.
Nine causes, ordered from the ones you can check yourself to the ones that need a technician, with what each typically costs to put right.
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An air conditioner that runs continuously without cooling is telling you something specific. In our experience across UAE villas and apartments, the cause falls into one of nine categories, and roughly half of them can be identified without any tools at all.
Work down this list in order. The checks are arranged so that the cheapest and most common causes come first.
Check these yourself first
- Thermostat setting and mode. Obvious, but genuinely common: the unit is in fan mode, or the setpoint has been nudged. Set it to cool and to 22°C and listen for the outdoor unit starting.
- Filter condition. Pull the return filter. If light does not pass through it easily, that alone can cost you a third of your cooling.
- Outdoor unit clearance. Anything stacked against the condenser, or a unit boxed into a service cupboard with no ventilation, will strangle heat rejection.
- Supply grilles. Closed or furniture-blocked grilles unbalance the whole system and can freeze the coil.
- Ice. Visible ice on the copper pipework or the indoor coil means airflow or refrigerant is wrong. Switch the system to fan only for two hours to defrost before anyone inspects it.
Causes that need a technician
| Cause | How it presents | Typical remedy |
|---|---|---|
| Low refrigerant charge | Cools weakly, ices at the coil, longer run times | Leak detection, brazed repair, vacuum, weighed recharge |
| Fouled evaporator coil | Weak airflow at grilles, musty smell | Chemical coil clean |
| Fouled condenser coil | Outdoor unit hot and loud, poor cooling in afternoon peak | Condenser wash and coil treatment |
| Failed capacitor | Compressor hums but will not start | Capacitor replacement |
| Failed fan motor | Airflow weak or absent despite unit running | Motor or blower replacement |
| Duct leakage | Some rooms cold, others warm; system runs constantly | Duct inspection, sealing, balancing |
| Compressor failure | No cooling at all, breaker may trip | Compressor replacement or unit renewal |
| Undersized system | Never quite reaches setpoint on the hottest days | Heat load assessment, capacity correction |
| Control or sensor fault | Erratic behavior, wrong temperatures reported | Sensor calibration or board replacement |
How a technician should diagnose it
The distinction between a good visit and a bad one is measurement. A technician who arrives, adds refrigerant and leaves has not diagnosed anything. Refrigerant sits in a sealed circuit and is not consumed, so if the charge is low there is a leak. Topping it up buys a few months and then you pay again.
What should happen instead: supply and return air temperatures measured at the coil, suction and discharge pressures read, superheat and subcooling calculated, current draw compared against the nameplate, and static pressure checked if airflow is suspect. Those five numbers separate almost all of the causes above from one another.
Ask for the readings. If the answer is vague, get a second opinion before authorizing any part replacement.
When repair stops making sense
As a rule of thumb, if the quoted repair exceeds 40 per cent of the cost of a comparable new unit, and the system is past eight years old, replacement is usually the better decision. Compressor failure on an older unit almost always falls into that category.
The counter-argument is disruption. On a ducted system, replacement means access, pipework and commissioning, so a well-executed repair on a seven-year-old unit can be entirely rational. A technician who will not discuss both options honestly is not giving you advice, they are giving you a quotation.
Common questions
Almost always a heat-rejection problem: a fouled or poorly ventilated condenser, or a system marginally undersized for the peak load. At night the ambient drops and the system copes; at 45°C it cannot.
It can be, but restricted airflow causes it just as often. A blocked filter, a dirty coil or closed grilles all drop the evaporator below freezing. Defrost first, then diagnose.
A correctly sized system should pull a closed, shaded room down by four to five degrees within the first hour. If it takes all afternoon, something in the list above is wrong.
Usually a failing compressor drawing locked-rotor current, a shorted fan motor, or a genuine electrical fault. Stop resetting the breaker repeatedly and have it tested, because repeated restarts under fault conditions do further damage.
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