When it gets cold (< 20 F), the driver door on my 2001 Audi A6 doesn’t latch closed.
What can I do? Spray WD40 on it?
You have the right idea. Spray wd-40 or any good penetrating oil into the latch. You are shooting blind, so aim in as many directions as possible. It has worked for me on several occasions.
If the striker-to-latch adjustment is borderline out of adjustment (latch is on door; striker is on body), temp change might be all it takes to throw off adjustment, but first get some brake cleaner or degreaser spray (Pep boys, etc) and try to spray out all the old grit/crud in latch. If you have a source of compressed air that’ll help; if not, apply some degreaser/brake cleaner in latch mechanism, get a skinny
screwdriver and w/door open and unlocked pull door handle like you’re opening door and open latch mechanism itself w/screwdriver. Repeat over and over till latch mechanism looks clean. Let dry awhile. Now apply lubricant. If no good you may have to pay someone to adjust striker- can’t be much; or maybe you’ll need new latch, hope not. Good luck.
Sometimes, locking the door first will make it latch.
Firstly; has this car had body work done? especially on the side of the door which is not latching?
Secondly: you need to closely look at the door when it is closed. does the seam between the door and the car side close evenly, with the same amount of gaps all around? is the door closing quietly?
If the door has uneven gaps between the seams then the hinges may be worn out. (if the hinges are worn the door latch can’t catch correctly to latch.)
If the door hinges are still tight and good, then the latch itself may be worn out, (or most likely) dirty, cruddy and clogged with gunk. as mentioned a good cleaning and relubricating with a powdered graphite lube will help the mechanism.
BUT, all the cleaning of the lock and shining up the mechanism wont help if the hinges or latch are worn out.