Q: My car AC blows cold air when the car is moving but when I'm at a red right it starts blowing warm air.

When the car is moving, versus stopped, there is more air flow over the condenser (the heat exchanger in front of the radiator) and that airflow is essential to system operation. To maintain airflow when the car is in stop and go traffic, or is completely stopped, condenser fans will come on automatically. If the fans don’t come on, the circumstance you have could occur. You might also have a low state of refrigerant charge. Other possibilities are that the compressor clutch is not cycling as intended or that the condenser might be blocked or dirty. And, of course, you could have a combination of these issues. To get this resolved, the best thing to do is request an air conditioning system inspection. The responding certified mechanic will come to your location, pinpoint the fault and get this resolved for you. Once diagnosed, the mechanic will be in a position to give you a repair strategy to get the A/C system fully functioning and he or she will estimate the cost of the required repair. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.