When I shut off my 2015 GMC Sierra, the round cigarette-lighter outlet on the dash still has power.
I noticed it while getting familiar with the truck. I’d recently found a USB port tucked inside the glove box and a household-style outlet above the round socket. Now I had another question: why is this outlet still on when the truck isn’t running?
I checked the owner’s manual. In my Sierra, that front outlet is supposed to work this way. What matters now is knowing what I can safely leave plugged into it and what might slowly drain the battery.
Why my Sierra’s front outlet stays on
GMC says the 12-volt outlets on the center stack and in front of the cupholders are powered at all times in the 2015 Sierra.
The round socket on the front of my dash is below the household-style outlet. I’ve confirmed that it still has power when my truck isn’t running, which matches what GMC says about the center-stack outlets.
The engine doesn’t need to be running for that socket to supply power. If I leave a charger plugged into it, I can’t assume the truck will shut off the charger when I turn the key off.
Does that mean something is wrong with my truck?
No. An outlet that stays powered after shutdown might seem like an electrical fault if you expect everything on the dash to go dark. For this front outlet, it’s normal.
It can be useful. I could charge a phone while parked without leaving the engine running. But the power has to come from somewhere, and with the engine off, that somewhere is the truck’s battery.
That’s why I treat an always-on outlet differently from one that switches off when I park.
Could a phone charger drain the battery?
Yes, if something plugged into the outlet keeps using power long enough. How quickly that becomes a problem depends on the accessory and the battery.
A phone actively charging draws more power than an empty adapter with a small indicator light. A dash camera recording while parked is another situation entirely. The condition of the truck’s battery matters too.
I wouldn’t tell somebody that leaving a little phone charger plugged in overnight will definitely leave them stranded. I also wouldn’t assume it’s harmless to leave accessories plugged in for days just because they’re small.
GMC warns in the 2015 Sierra owner’s manual that equipment left plugged in for an extended time while the vehicle is off can drain the battery. The simple habit for me is to unplug the charger when I’m done with it.
What if there’s nothing plugged into the outlet?
The fact that an outlet has power doesn’t mean the outlet itself is pulling the kind of power a charger or other accessory would. The concern is what I connect to it and leave running.
If my truck ever develops a battery drain while it’s parked, I’d look at anything left plugged in before blaming the socket for being powered. If removing the accessories doesn’t help, the cause needs a proper battery and electrical diagnosis.
Do all the outlets in a 2015 Sierra stay on?
No, and this is where it’s easy to get mixed up.
GMC says the outlets on the center stack and in front of the cupholders are powered all the time. It says outlets inside the storage area and on the rear of the console receive power with the ignition in ON/RUN or ACC/ACCESSORY, or while retained accessory power is active.
Retained accessory power lets certain electrical features work for a limited time after the ignition is turned off. An outlet connected that way could still work right after parking and then stop later.
So I wouldn’t test one outlet in a Sierra and assume every socket in the truck works the same way. I’d check the location of the one I actually plan to use.
Is it okay to charge my phone with the truck off?
I would use the front 12-volt outlet to charge a phone while parked, but I’d keep an eye on how long I’m doing it. Charging during a short stop is different from leaving a phone and charger connected while the truck sits for several days.
If the battery is weak or the truck has been hard to start, I wouldn’t use the parked truck as a phone charger until I understood what was going on with the battery.
And I’d unplug the charger before walking away. That’s easier than trying to guess exactly how much power every accessory draws over time.
What about a dash camera or something I want running overnight?
That takes more thought than charging a phone for a few minutes.
An always-powered outlet may let a dash camera keep recording while the Sierra is parked. But “it keeps running” only tells me that the camera has power. It doesn’t tell me how long the battery can support it or whether the camera will stop before the battery gets too low to start the truck.
The same concern applies to a cooler or any other accessory meant to run for hours. I’d check the device’s power draw and whether it has battery protection before leaving it connected.
If I wanted dependable parking-mode recording, I’d look for a setup designed to shut off at a chosen battery voltage. I wouldn’t leave an ordinary plug-in camera running indefinitely and hope for the best.
The round outlet and the household-style outlet have different limits
My Sierra has a household-style outlet just above the round 12-volt socket. Being so close together makes them look like two ways to do the same job, but they aren’t.
The round socket is for a 12-volt DC accessory plug, like a car phone charger. GMC lists a 15-amp maximum for the 12-volt accessory outlets.
The household-style outlet supplies AC power for compatible devices with a regular plug. GMC rates that one at 150 watts. I broke down what I can plug into the household-style outlet in my Sierra because its familiar shape doesn’t tell you how limited its power is.
I wouldn’t carry a wattage or amp rating from one socket over to the other. When I buy a charger or accessory, I want its plug and power needs to match the outlet I intend to use.
Is the glove-box USB port always on too?
I wouldn’t assume so just because my round front outlet stays powered. The ports serve different purposes and don’t necessarily follow the same power schedule.
Finding the USB port hidden in my truck’s glove box is what started me looking more closely at where this Sierra supplies power. For anything I might leave plugged into that USB port, I’d check whether the connected device is still charging after I shut off the truck.
The front cigarette-lighter socket is the one I’ve personally confirmed stays powered when the truck isn’t running.
What if your cigarette-lighter outlet doesn’t work?
If yours is a 2015 Sierra with a front center-stack 12-volt outlet, GMC says that outlet should have power at all times. I’d start by trying a different car charger that I know works. A bad charger can look exactly like a dead socket.
I’d also make sure the plug fits and stays seated. Some accessory plugs make poor contact even though they seem to be pushed all the way in.
If nothing works in the outlet, the fuse is worth checking using the fuse information for your truck’s equipment in the owner’s manual. I wouldn’t replace a blown fuse with one rated higher. If the correct replacement blows again, the underlying problem needs to be found.
I’d be careful around anything stuck inside the socket, too. Since my front outlet has power with the truck off, I wouldn’t reach into it with a metal object.
For now, I’m glad I noticed how my truck works. The front round outlet gives me a convenient place to charge something while parked. I just know that turning off the engine won’t turn off whatever I leave plugged into it.
About the author: Daxon Weaver owns the 2015 GMC Sierra 4×4 in this article. He writes about the features, repairs, and everyday questions that come up while driving his own vehicles.
