Trying to replace an external door threshold. I have tried removing the screws with a screw gun with no luck. I have tried using an impact driver and hammer with no luck. The heads of the screws are rusty. I can't tell if they are rusted in place or really tight. I thought for sure the impact driver would work but they don't budge. Any ideas? I have added a couple of pictures of the screws causing the grief. Any help would be greatly appreciated.
Judging by the picture and your description I'd say those are indeed the adjustment screws that raise and lower the rail of threshold (the part that makes the seal with the bottom of the door). If the door is installed normally removing these will not help you remove the threshold. It will be attached to the jamb via screws that run through the jamb into the threshold and it will be silicon-ed down to the rough opening. Here's a link that shows the basic anatomy of removing and installing a threshold in place. The guy clearly doesn't know what he's doing but the pictures are pretty good. It is certainly debatable whether or not its more efficient to remove/replace the threshold in place or just pull the whole jamb and replace the threshold with the jamb out.