Why is my aircon cold then warm?
A gradual fade, with cooling strong at first then weakening over the run, points to the coil freezing up behind the cover. A sudden cut while the indoor fan keeps blowing points to the outdoor unit dropping out. A random cycle that ignores the room points to sensor drift.
By Team Snowflake | Reviewed 30 May 2026
1. Coil freeze-up
You feel strong, cold air when the unit first starts, then over the run the airflow thins and the air turns warmer. Behind the cover, ice is building across the indoor coil, choking the air it should be cooling. The longer the unit runs, the worse it gets, and water often drips heavily once you switch off and the ice melts.
How to tell
Watch how the cooling fails, not just that it fails. The airflow thins out as the air warms, both fading together and gradually. That weakening airflow is the tell. Unlike a sensor or outdoor-unit fault, where the airflow stays steady, here it drops off. To confirm, switch off and look for frost on the indoor pipe.
- Cooling is strong at first, then steadily weakens during the same run.
- Airflow from the vents thins out the longer the unit stays on.
- Heavier dripping or water around the indoor unit after you switch it off.
How we confirm it
We let the coil thaw fully, then check the filter and airflow path and measure refrigerant pressure to find what is starving the coil. If gas is low, we leak-test the joints before any top-up so the freeze does not return.
Stop the unit if airflow has dropped off and you suspect ice on the coil. Running the compressor against a frozen coil makes it work against blocked suction and accelerates wear. Do not restart until the coil has fully thawed and the cause is found.
2. Room sensor drift
The unit cools for a stretch, then winds down as if the room is already cold, while the room still feels warm. A small indoor coil or return-air sensor is reading too cold, so the board ends the cooling call early. The unit restarts later and repeats the same false rhythm.
How to tell
Sensor drift follows a regular rhythm. The unit eases off even though the room is still warm, then restarts on a similar schedule. Unlike freeze-up, airflow stays steady. Unlike an outdoor dropout, the outdoor unit keeps running. A sensor resistance check against real room temperature confirms the mismatch.
- The room warms back up faster than it should after the unit eases off.
- Cooling restarts on a regular rhythm that does not match how warm the room feels.
- Airflow stays steady throughout, with no thinning before the warm phase.
How we confirm it
We measure sensor resistance and compare it with the real room temperature. If the values are out of range, we replace the sensor and retest the full cycle.
Before replacing the sensor, check the filter and airflow path. Poor airflow can mimic drift, and skipping that check can send you to the wrong part.
3. Outdoor unit dropping out under load
Cooling cuts off suddenly a few minutes in while the indoor fan keeps blowing, leaving you with normal airflow but room-temperature air. The outdoor unit has dropped out under load, most often from a weak capacitor, an overloaded compressor, or a condenser-side fault. It tends to bite hardest in the heat of the afternoon when the system is working its hardest.
How to tell
Outdoor dropout is abrupt. The indoor fan keeps blowing room-temperature air while the outdoor unit goes quiet. Unlike freeze-up, airflow does not thin first. Unlike sensor drift, the outdoor unit stops. It may restart a few times, with each run shorter than the last.
- The outdoor unit starts, then goes quiet while the indoor unit keeps running.
- Cooling stops abruptly during a run rather than fading over many days.
- The pattern is worse in the heat of the afternoon.
How we confirm it
We run the outdoor unit under load and watch when it drops out. Then we test capacitor value, compressor current, and condenser-side behaviour. The repair follows the component that actually fails.
Avoid a gas top-up before outdoor startup, capacitor, and current readings are checked. The silent outdoor unit matters as much as room temperature.
Related reading
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