Why does the car vent suddenly blow hot air?

[BriellaLee]

The reasons why the car vent suddenly blows hot air: 1. The external circulation is turned on, as the incoming air passes through the engine compartment and carries the heat from the engine into the car through the air distribution box, causing the vent to blow hot air; 2. The car's heating system is turned on, leading to hot air from the vent. Simply adjust the vent's circulation mode or turn off the car's heating system switch. It is normal for the vent to still blow hot air after turning off the air conditioning. After turning off the air conditioning, just set the temperature to the lowest and switch the air conditioning to internal circulation mode. The car air conditioning cooling system consists of a compressor, condenser, receiver-drier, expansion valve, evaporator, and blower.

[SanAlice]

My old car had the same weird issue last week—suddenly blowing hot air from the AC vents while driving. The mechanic later told me there could be several reasons: the most common is the AC system automatically switching modes, like when it’s set to 25°C but the outside temperature drops to 20°C, so the computer decides heating is needed. Or maybe you accidentally bumped the temperature knob and turned it up. Another possibility is having the recirculation mode off, letting hot air from the engine bay seep in. The worst-case scenario is a stuck blend door, stuck in the heating position. I’d suggest first checking your AC settings and recirculation mode yourself. If that doesn’t fix it, head to the shop ASAP to inspect the blend door actuator—driving in summer without cold air is unbearable.

[James]

As an auto mechanic, I encounter car owners complaining about hot air from the AC almost daily. The first thing to check is whether your AC is manual or automatic. Faulty sensors in automatic AC systems often cause temperature misjudgment, triggering heating mode during summer. For mechanical issues, inspect whether the air blend door is stuck or if the actuator motor is burnt out. Also, don't overlook a leaking heater core valve, which allows engine coolant heat to transfer into AC ducts. The troubleshooting process is straightforward: restart the system, check for clogged cabin filters, then scan for trouble codes. Most issues can be resolved by simply replacing an actuator motor.