I want to know if it is possible to turn on the A/C unit using the OBD-II port in a car even if the engine/car is off. Is the OBD-II port capable of doing something like this, or is it only used to diagnose car issues? I am looking to work on a project and turning on the A/C while the car is off is the main thing hindering me from working further on this.

If this is possible, can someone briefly explain to me how I could do it, or if there is another way to turn on the A/C unit in a car even if the car is off?

Thanks for any help.

  • Welcome to Motor Vehicle Maintenance & Repair! Is the car completely off (ign in off position)? What are you considering "on" as far as the A/C is concerned? Commented Jul 30, 2022 at 17:48
  • @Pᴀᴜʟsᴛᴇʀ2 the car would be completely off
    – Bob
    Commented Jul 30, 2022 at 17:49

2 Answers 2


Since the car would be completely off, then it wouldn't make a difference if you could turn on the A/C. A/C compressors work off of the car's engine. If the engine isn't running, then your compressor isn't working and therefore there's no cold air being pumped out of the HVAC system.

You might be able to get the A/C turned on and working on an EV since they would work off of electricity only. To do this would take specialized equipment and software which could communicate with the car's computer.


Depends on the car, and depends on what you mean by "off". But yes, OBD II ports do have functionality beyond mere smog testing.

A number of systems in the car need to be powered up, such as the thermostat controls, the interior fan, and the radiator fans. So the car will need to be in "accessory" or "run". Running in true "off" is an impossibility, unless we are talking about an RV or "VanLife" build, in which case it could have a separate electrical system for the accommodation area, and hopefully a more efficient A/C than the typical automotive system.

Air conditioning takes a lotta lot of power, so the vast majority of cars use an A/C compressor driven off the engine's belt drive, which means the engine must be spinning.

Electric A/C is starting to show up, but it takes vastly more power than a common car battery can possibly deliver. Therefore the only way it can run off battery is from a large battery such as a hybrid or electric vehicle. Hybrids need to be able to run the A/C off the battery, since otherwise you'd be spinning up the gas engine just for that in city driving.

Lastly, if your goal is to protect a dog or baby in the car while you are away from the car, forget it. Good Samaritans will have no idea the A/C is on, and will smash windows to save the being. And the cops will come, and you will get a ticket and the Good Samaritans will have legal immunity. They couldn't have possibly known.

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