Why does the car air conditioner sometimes blow cold and sometimes hot?

[VonPeter]

The reasons why a car air conditioner sometimes blows cold and sometimes hot include: loose wiring of the compressor; poor contact of the pressure switch; poor contact of the relay; or a faulty compressor clutch. Additional information: 1. High-temperature gas cools into liquid: When the air conditioner is first turned on, the compressor starts working, compressing the gaseous refrigerant in the system into a high-temperature, high-pressure gas, which is then sent to the condenser for cooling. After cooling, it becomes a high-temperature, high-pressure liquid, which is dried and filtered through the dryer bottle, then passes through the expansion valve and evaporator before returning to the compressor. 2. High-temperature, high-pressure liquid expands: When the high-temperature, high-pressure liquid passes through the expansion valve, it undergoes a drastic change, turning into a low-temperature, low-pressure liquid. These low-temperature, low-pressure liquids cause the temperature of the evaporator to drop rapidly, so the air blown by the air conditioner motor becomes cold after passing through the evaporator.

[August]

I've been driving and repairing cars for over a decade, and the most common reason for air conditioning that alternates between cool and hot is insufficient cooling or system issues. For example, it could be due to insufficient refrigerant causing unstable pressure when the compressor is working, which makes you feel like the cold air disappears quickly and turns into hot air. Or, the compressor itself might be faulty, with wear or bearing problems, causing it to stop and start intermittently, unable to control the temperature. Blocked pipelines are also a major issue—if the condenser is clogged, poor heat dissipation reduces cooling efficiency. Additionally, if the electric fan is broken and not spinning, or the fan clutch is stuck, the temperature of the radiator rises, and the air conditioning automatically switches to hot air to protect the system. Don’t overlook the expansion valve’s unstable regulation affecting refrigerant flow. I recommend first checking the refrigerant level and pressure for leaks. It’s best to go to a professional shop for a tool-based inspection rather than trying to fix it yourself, as this affects the car’s overall performance and safety. Regular maintenance of the air conditioning system while driving can prevent these troubles—don’t wait until the scorching summer heat leaves you stranded before regretting it.

[OAriel]

As an automotive electronics enthusiast, intermittent cooling and heating from the AC usually indicates a control system malfunction. The temperature sensor might be faulty, falsely reporting cabin temperature and causing the ECU to incorrectly cycle the compressor. A problematic thermostat controller or AC module can also cause this—like internal circuit shorts or poor connections disrupting the refrigeration command. Aged relays with burnt contacts may cut off power supply, forcing the AC to default to heat mode. I've encountered loose plugs too; unstable wiring connections on hot days can suddenly switch cold air to hot. For diagnosis, use an OBD scanner to read trouble codes and quickly pinpoint the issue—most cars have this port. Also, abnormal capacitor charging/discharging can cause system fluctuations; remember to clean circuits during maintenance to reduce dust. From personal experience, don't push through on highways—stop promptly to check, as it's a major safety risk. Learning basic repair skills saves money, but always consult a reliable mechanic to avoid causing more damage.

[TessaLee]

I often take my wife and kids out for drives in our family car, but we frequently encounter issues with unstable air conditioning, which really affects comfort. The root cause is often inadequate maintenance, such as a dirty or clogged air filter disrupting airflow and causing inconsistent cooling performance. After prolonged exposure to the sun, the cabin temperature gets too high, and when you turn on the AC, the compressor may overload and trigger a protection mechanism, shutting down and blowing hot air instead. Another issue is constantly switching between recirculation and fresh air modes, which can make the system sluggish—especially in city traffic with frequent AC cycling. A simple fix is to clean the AC system yourself, starting with the filter. If that doesn’t work, try recharging the refrigerant. I did this last month, and the improvement was immediate—a small expense that saved me from major repairs. When driving daily, avoid blasting the AC at full power in extreme heat, as this can cause the evaporator to freeze up and block airflow. Adjust the fan speed moderately. Overall, make it a habit to check the system every season—it saves hassle and money. Don’t slack off this summer and suffer the consequences. Safety first—if you notice any abnormalities, drive cautiously and avoid rushing onto the highway.