You don't fix it you remove it, the locking mechanism that is. I removed mine and it takes about 15-20 min. to remove it from the gun. Its easy even with basic mechanical/gunsmithing skills, nothing to it. It does not leave a hole in the frame all you are removing is the internal "flag" which is what disables (locks the hammer) the gun when locked.
While I did remove mine and the gun functions 100 percent reliably its really not needed and the so called cases of guns locking up under recoil are vastly exagerated.
Another internet myth basically. But its so easy to remove why not do it, and its a good time to clean out the insides while you are at it. And again .....It DOES NOT leave an open hole... the weapon looks EXACTLY the same after the locking mechanism is removed.
Here's how step by step, great video.
YouTube - S&W Internal Lock Removal