The drivers side door won't open/unlock with the key, the door handle works and the inside mechanism works also.

But when the car is locked from the inside, its impossible to open it from the outside.

What should I look into replacing?

 

This isn't trick question is it? Does the lock cylinder turn back and forth? Did you go into the door to make sure nothing came loose?

 

Yes the cylinder goes back and forth. I didn't want to take the door panel off before I had the parts that I needed. So I wanted to get a sense for what it could be.

 

If you don't take the door panel off to find out what is the problem then how do you plan on ordering parts to fix it? What if you don't need parts and it's an easy fix?

You can take the door panel off in less than 5 minutes.

 

Well if it's a common issue then I'd like to know what to look for...