Have a problem with a door that will not lock or latch properly? These difficulties fall into two general categories: those related to the lock set or key, and those related to the door's frame and latch assembly.
If the key turns but does not operate the lock bolt, the lock mechanism is broken. Remove the mechanism to have it repaired, or replace the lock set yourself.
To remove a mortise lock-set cylinder for repair (see illustration for How to Tighten a Loose Doorknob), remove the deadbolt knob by loosening the setscrew in the sleeve of the knob using a standard screwdriver or Allen (hex) wrench, or by removing any mounting screws. Then, on the faceplate, loosen the setscrew opposite the lock cylinder and unscrew the lock cylinder.
To remove the rest of a mortise lock set, take off the interior knob, the exterior handle and any trim. Remove the faceplate and strike plate by removing their screws.
To replace a mortise lock set with one that has the same dimensions, reverse the procedure in step 3, making sure to follow the detailed instructions that the new unit will probably include.
To remove cylinder and tubular lock sets, locate a slot on the shaft of the knob. Depress and hold down the protruding spring clip with a screwdriver (or a thumbnail) as you pull the knob straight out (see A). Or, if there is no slot, loosen the two machine screws in the face of the rose. If there's a mounting plate, loosen (or sometimes remove) the two machine screws to take out the cylinder (see B).
To replace the lock set, reverse the procedure in step 5. If you cannot reuse the strike plate (or prefer not to), unscrew it. Position the new one to check the fit in the mortise (recess).
If the mortise is too large, install the strike plate temporarily and fill the excess mortise with wood putty. When the putty is hard, remove the plate to sand the putty smooth and touch up the finish (use either primer and paint or stain and varnish).
If the mortise is too small, trace the plate's perimeter on the wood and use a sharp chisel to extend the mortise. First make vertical cuts at the perimeter with the bevel side facing inward. Then, with the bevel flat on the already mortised area and the chisel at a low angle, chisel out wood up to the perimeter cut.
If a replacement key won't turn, it may have been cut poorly. Check the lock set's operation using the original key. If the replacement key is faulty, take it back to the store where you had it cut and ask for another.
If the key turns but only with effort, lubricate the lock mechanism with graphite powder or silicone spray by injecting the lubricant into the keyway. Work the key or thumb latch back and forth to distribute the lubricant.
If lubricating fails, the tumblers in the lock cylinder might be damaged. Remove the lock cylinder or lock set and bring it to a locksmith for a minor repair.
If the key either does not fit into the lock or won't turn, and you've been having wet, freezing weather, the lock is likely frozen. Spray a lock de-icer into the key slot or warm it with a hair dryer.
If a latch bolt doesn't extend fully, the mortise for it might not be deep enough. Remove the strike plate and bore the hole deeper; the hole has a 7/8-inch (22-mm) diameter, so you'll need a spade drill bit that size.
If you have to push against the door to make it latch, weather-stripping may be pressing against the door. Reposition the weather-stripping; adjust the strike plate's position; or remove the strike plate, put it in a vise to file off the offending portion, and reinstall it.
If a latch bolt bumps into the bottom edge of the hole in the strike plate, tighten the hinges (see How to Fix a Binding Door). If that doesn't fix the problem, adjust or file the strike plate as described in step 6, above.