The first thing I would do is spray graphite in the lock and see it helps.

 

As myke212 mentioned; graphite is a good lubricant for lock barrels. You may find someone has squirted something else in there before. There are many solvent cleaners, similar to switch contact cleaner that will help remove the junk before relubricating with something more suitable (graphite or a very light oil that will not thicken).

 

I would try a good locksmith. As others said, there could have been unsuitable lubricants used. Oils tend to attract dust/dirt that then hardens like varnish and that will need to be cleaned out. Powdered graphite is best.

You did not provide your location in the US so we cannot know if you are in an area where freezing could be the problem. A lock deicer might help. Heating the key with a flame before inserting can work in a pinch.

 

I had a similar issue with the trunk on a Suzuki.
Kept working it with the key and some spray solvent/lube.
It freed the lock mechanism. And now I can use the key to open the trunk.
Think your issue might be the same????

 

Thank you for all of your replies! I live in Boston, MA. This problem only occurs when it's below freezing point. We thing there might be water and then ice in the lock.
Now I tried spraying some WD40 into the keyhole and put some lubricant outside. It will get cold again this week. I will see if it works.

 

WD40 will do a good job gumming up a lock.
It should help temporarily but I would highly recommend getting the proper stuff and giving it a clean when the weather warms up for you. It's worth it in the long run.

 

Still not working this morning. It was 28 degrees.
I had to blow in some air manually into the keyhole. There must be some water and then ice there.
Any way to permanently solve this? Thanks!

 

I think the solution already offered is probably the best you will get...

Clean it.
Lubricate it with something suitable such as graphite or, if you prefer, a light lock oil.
If it still won't free up, replace the lock.

Don't put WD40 in the new one!

 

I've had WD-40 "freeze" on me in some instances, (but never @ 28 F). WD40 does displace water though. Go with the graphite, I've even used a powdered graphite when & if convenient.

I also had a lock go inoperable when someone attempted to pick it therefore damaging its internal components.

I make it a habit to use the manual locks every so often to keep them working. I'm really against the the manufactures decision to remove the manual locks disabling the ability to get inside upon electrical failure.

I tried calling HYUN dealer, but they need to charge $140 to diagnosis the problem. Thanks

Gotta love these pure profit "diagnostic fees" in most industries.

 

Most dealers charge 1hrs labor for touching your car. Around here that's $140. I have one dealer that only charges .5hrs labor for many issues. It pays to shop dealers. At our dealers the fee is applied to any repair total.