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Summary
  • One main reason why people turn off the A/C is because drivers believe that keeping the A/C turned on can damage the engine when the vehicle is started. However, this might only be an issue with older vehicles.
  • Some users also believe that turning off the A/C and leaving the blower on before leaving the vehicle allows water inside the vehicle’s condenser to evaporate, which leads to less mold and bacteria buildup.
  • Overall, the practice of turning off the A/C before shutting down the engine might be more relevant for older vehicles with carburetor engines.

Overall, the practice of turning off the A/C before shutting down the engine might be more relevant for older vehicles with carburetor engines. Also, whether you start the car with the A/C off or on doesn’t matter that much. But it’s common for drivers to turn off their vehicle’s air-conditioning (A/C) system before shutting down their engine. Just like turning off the lights before leaving a room, turning off the A/C feels like the right thing to do.

Whether you start the car with the A/C off or on doesn’t matter that much.

Richard McCuistian, ASE Certified Master Automobile Technician

However, some people wonder whether the practice can be done in reverse. Is it really bad to leave the air conditioner on in a car? Meanwhile, others even ask whether it’s even worth turning off the A/C at all. We’ll go over possible reasons why people do this and give you a definitive answer to the question of whether you should turn off the A/C before turning off your car.

Reasons Why You Should Turn Off the A/C Before the Engine

Some people say that this practice helps preserve the vehicle’s engine and prevents damage to the vehicle’s A/C system. However, others believe that it’s unnecessary and that leaving the A/C on has no detrimental effects. Let’s examine the facts. 

To Prevent Engine Wear

One main reason why people turn off the A/C before turning off the car is because drivers believe that keeping the A/C turned on can damage the engine when the vehicle is started. This is because the A/C compressor requires engine power. When the A/C is left on, the compressor will activate immediately when the vehicle’s engine is turned on. During startup, the compressor’s extra load can put excess stress on the cold engine and accelerate wear.

But this line of thinking doesn’t consider the fact that some vehicles with a compressor mounted down low will engage the compressor while the engine is spinning (regardless of whether the A/C is on or not) to work settled refrigerant oil out of the compressor piston chambers. Ford calls this “C.A.S.S.” for Compressor Anti Slugging Strategy.

Some drivers wait for the engines to warm up before driving to allow oil to warm up and circulate to prevent accelerated wear on its components. But most drivers just fire up and head out, and this is what engines are designed to do. The caveat is that switching the engine off before it’s had a chance to warm up tends to form sludge in the crankcase, so make sure you don’t shut the engine off until it’s warm.

If the weather isn’t freezing, most vehicles can be driven as soon as their engines are started. If vehicle engines can handle the load from moving as soon as they’re started, then they can likely handle the extra load from the A/C compressor without running into issues.

To Prevent Bacteria and Mold Buildup

Some vehicle owners also believe that turning off the A/C and leaving the blower on before leaving the vehicle allows water on the outside of the condenser fins and tubes. Since bacteria and mold don’t grow as effectively in dry environments, this practice might inhibit bacteria and mold buildup, which can prevent your vehicle cabin from smelling funky or unpleasant.

Some Cadillac platforms have a strategy of running the blower motor after the vehicle has been shut off for this very reason. If you own one of these and hear your blower motor running for a short time after you’ve left the vehicle, it’s probably programmed into the electronics for it to do that.

Just remember that bacteria and mold buildup in your vehicle’s A/C system is an entirely different issue. Turning off your A/C might not be a definitive way to prevent or solve this problem.

To Preserve Battery Power

The blower remains on even when the engine isn’t running as long as your vehicle’s ignition is in the “on” position. This will use up power, which can eventually drain your vehicle’s battery. This won’t be a big concern if your battery is in good condition and you’re not going to keep the key in the “on” position running the blower for a long time.

Again, overall, the practice of turning off the A/C before shutting down the engine might be more relevant for older vehicles. If you’re in a modern direct-injection vehicle, your engine can certainly handle the extra load from the A/C compressor even after a cold start.

The choice of whether you’ll turn off the A/C before the engine might boil down to personal preference. If you prefer to be on the side of caution, then there’s no harm in doing it. However, in most vehicles, leaving the A/C on until the engine is turned off is unlikely to cause any significant harm.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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