An A/C that is ice cold at first and then turns warm, or only cools at highway speed, is a common complaint in hot weather. The pattern helps point to the cause.
Possible causes
- Evaporator freezing. Ice builds up on the coil and blocks the air. It melts once the system is switched off, so the air is cold again after a rest.
- Low refrigerant. The system cools weakly, and cooling drops further when the engine is hot or the car is idling.
- Condenser fan not working. Cooling is fine at speed but gets warm in traffic.
- Dirty condenser. Dust and debris at the front of the car stop heat from leaving the system.
- Failing compressor clutch. It cuts in and out when it gets hot.
- Faulty thermostat sensor or control. The system switches off when it should not.
Pay attention to when it happens
- Warm in traffic but cold on the highway: look at the fans and condenser.
- Cold, then weak airflow after ten minutes: look at icing.
- Cooling gets worse week by week: look at refrigerant level and leaks.
A short note of when the problem appears is very useful for the technician, and it is often enough to narrow down the fault before any parts are touched.