What are the impacts of running the car's air conditioning without turning off the engine in summer?

[IsaacLee]

The impacts of running the car's air conditioning without turning off the engine in summer include: causing engine carbon buildup, which may endanger lives. Causing engine carbon buildup: When the engine is idling, the throttle is in a critically closed state, resulting in minimal air intake. At this time, the engine operates in a rich fuel injection state. Due to the overly rich air-fuel mixture, carbon buildup tends to be relatively more significant. If the air conditioning is also running, the engine needs to output a certain amount of power. To avoid stalling, the engine's RPM will increase slightly, typically by about 50 RPM. This low-speed operation under load can lead to more carbon buildup. May endanger lives: If the car idles with the air conditioning on for an extended period, the overly rich air-fuel mixture causes incomplete combustion, leading to the generation of a large amount of carbon monoxide. Carbon monoxide is slightly lighter than air and can accumulate around the car in windless conditions. It may be reabsorbed into the air conditioning system, causing occupants inside the car to suffer from oxygen deprivation and suffocation, posing a serious risk.

[StKira]

With over a decade of car repair experience, the most common issue I see in summer is vehicles that keep running with AC on without shutting down the engine. This practice severely damages the engine—idling already results in poor cooling efficiency, and the continuous load from the AC compressor pushes the engine further, causing the temperature gauge to spike in no time. The compressor itself can't handle four to five hours of non-stop operation either. Last summer alone, I repaired three cars with burnt-out compressors, all belonging to ride-hailing drivers who left their engines running for convenience. What's worse, prolonged idling with AC on leads to carbon buildup, and clogged fuel injectors end up wasting more fuel. The scariest part is exhaust gas backflow—doing this in an enclosed garage can be deadly. My advice: if you're waiting at a red light for more than three minutes, turn off the engine. It's better for your car and safer for you.

[LeCayden]

I did exactly that last week—waited in the car with the AC on for three hours in scorching heat. Later, the car started shaking violently, and I smelled something burning. My mechanic friend said idling with the AC not only guzzles gas but also drains the battery fast. He explained that prolonged low-RPM operation leads to incomplete fuel combustion, leaving thick black carbon deposits in the exhaust pipe. The worst part? The AC evaporator stays damp, causing the entire AC system to mold afterward. Now, if I wait over 20 minutes, I turn off the AC and roll down the windows. That $500 I spent last year cleaning the AC ducts was a costly lesson learned.

[VanMiles]

Seasoned drivers know that idling with the AC on in summer is the most damaging to cars. The cooling system takes the first hit—the radiator fan spins like crazy but still can't dissipate the heatwave in the engine bay. Next is the AC compressor, whose belt is prone to snapping under continuous operation. The battery voltage gets drained to just 11 volts by the stereo and AC, making it hard to even start the engine. The biggest waste is fuel consumption: tests show a 2.0L car idling with AC burns two liters per hour, worse than cruising. Last year, my compressor gave out because of this, costing over 3,000 RMB to fix. Now, if I'm waiting more than ten minutes, I turn off the engine and find some shade.