There could be a few possible reasons for the issue you described with your car's air conditioning system. Here are a few potential causes:
Low refrigerant level: The refrigerant in your car's AC system is responsible for cooling the air. If the refrigerant level is low, it can result in the AC not blowing cool air consistently. Over time, refrigerant can gradually leak from the system, leading to reduced cooling performance. The AC may work normally after being shut off overnight because the system has had time to equalize, but the problem reoccurs during extended usage.
AC compressor issues: The AC compressor is a vital component that pressurizes the refrigerant and circulates it through the system. If the compressor is failing or overheating after prolonged use, it can result in intermittent cooling performance. When the car is left off overnight, the compressor cools down, allowing it to function normally initially but fail again when it heats up during extended use.
Cooling fan problems: The cooling fan in the engine compartment helps to dissipate heat from the AC system. If the fan is not functioning correctly or is not running at the necessary speed, it can cause the AC system to struggle in hot conditions. This issue may appear after several hours of driving, but when the car is parked overnight, the fan cools down, allowing it to work properly again initially.
It's important to have a qualified mechanic inspect your car's AC system to diagnose the specific cause of the issue. They can perform tests, check the refrigerant levels, examine the compressor and cooling fan, and identify any other potential problems that may be affecting the cooling performance.