Why is one of my new tires losing psi?

I got 4 new tires about 2 maybe 3 weeks ago on my 2009 Honda accord coupe and when I was leaving work and pulled out of my parking spot I noticed the low tire pressure light came on and when I got home I checked the psi and the tire has around 26-28 psi. It didn't look like any damage was done to it anywhere.
Is this normal? Should I just air it back up and put something like fix-a-flat in it?
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You really should take it somewhere to have it checked properly. Putting in a fixative is only a temporary measure, and it WILL go down again - sooner or later!

It's not normal for a new tire to drop pressure like that. You should re-inflate the tire to its proper pressure then take it back to whoever installed the tires to have them check the tire and its valve stem for a leak.

Don't put fix-a-flat in it. Not only is that stuff only suitable as a temporary fix, if you use it the tire shop won't be able to determine where the leak is coming from so they won't, for example, change out the valve stem for free if they can't tell it's leaking. If you come back in 3 months and the valve stem or tire is leaking again once the fix-a-flat stops working, they likely won't fix the valve stem or look at the tire for free like they will right now since they just installed it.

Also you stated that the low pressure light came on, this means you have a tire pressure system on the vehicle. Products like Fix-a-flat may stop your leak but will probably clog and ruin the sensor in the tire. So when you do go back in to the tire dealer more than likely you will have to replace the sensor. That can be sometime in the $50-$100 range. Refill the tire and if it starts to lose air again take it back to the shop and have them look at it.

FIX A FLAT? NO… Go back to the tire store,… You probably have a " bead leak ",( between the tire & wheel) or maybe even a bad tire…( not very likely )

Fix a flat? Are zombies chasing you and this is your last resort?

If not, actually fix the issue.

You might have run over a nail or something else small punctured the tire. Either that or the bead is not sealing around the rim very well. Take the car back to the tire shop and have them check it out.

They didn't change the valve stem.