I looked around online, and found a few different solutions to the problem. One involved tapping the key with a hammer to get it to work, another involved drilling out the old cylinder to install a new one.
The biggest problem I found was that each solution was either temporary, expensive, or involved getting a new key. This means that you would end up with one key for the doors, and a different one for the ignition - not the solution I wanted.
This instructable will show you how I solved the problem. I ended up with a new ignition lock cylinder, but continue to use the original, programmed key. The total cost of the repair was the price of the part - $72.99 with tax. The whole job took me about 4 hours, but I had to figure out how a lock works without much help. If I were to do it again, it would probably only take 2 hours.
The repair involves changing the ignition lock cylinder for a new one, and reprogramming the computer to accept the new keys. Ford has a passive anti-theft system (PATS) where each key has a chip, and they have to be programmed in. No problem if you've got two functional keys, because you can add new ones, but you have to be able to turn the key in the ignition to do so.
Step 1: Stuff You're Gonna Need
#2 Philips head screwdriver
T20 Torx screwdriver
8mm socket and driver
small screwdriver (I used a small torx screwdriver)
small flathead screwdriver
clamp or vise
clean, light-coloured work surface
snap-ring pliers (might make the job easier, but I did it without them)
power drill and grinding bit
replacement lock cylinder (comes with 2 chipped keys)
Step 2: So What the Heck Is Going Wrong???
Besides this, I found page after page of people complaining about this problem in their Focus, and quite a few F-150 owners, too. The problem seems to be that the tumblers get stuck, and the key won't turn in the ignition any more.
Although I bought mine independently, there is a helpful fellow online who appears to be a good place to get ahold of a replacement. Check out http://www.focusfanatics.com/forum/showthread.php?t=121739 for more info.
Step 3: Disconnect the Battery
Now go have a coffee or something. Seriously. There's a capacitor in most airbag systems that needs a couple minutes to discharge. Better safe than sorry.
(Okay, these are photos of my Hyundai Accent, not the Focus, but the idea's the same).
Step 4: Get at the Ignition Cylinder
The instructions at http://www.mbremerart.com/misc/Focus%20Manua/2003%20Focus%20Ignition%20Lock%20Cylinder.htm give a good idea of how to go about removing everything.
First, the lower dash panel cover comes off. Undo 4 8mm bolts, and then pull gently to release the clip that fastens the left-hand side. If your model is the wagon (as shown here), the lower dash panel will remain attached by the trunk release handle and the OBD2 diagnostic port. You don't need to disconnect these - just let the panel rest on the floor.
Next, insert a thin-bladed flathead screw driver into the slots on either side of the upper steering column cover, and release the tabs holding it down. There is one on either side of the steering wheel. If your car has it, you have to unclip the audio control connector, but mine was not equipped with this, so it was a bit easier.
Now release the steering wheel lever, and undo the three T20 Torx screws holding on the steering column cover. Pull down and shuffle it out of the way, and place to the side. Try to keep the screws organized, and you'll have an easier time putting everything back together.
Finally, turn the wheel a quarter turn to the right, and you'll have a clear view of the lock cylinder.
Step 5: Remove the Defective Cylinder.
Insert the key into the lock, and turn it to the ACC position. Now use a small screwdriver, pick tool, or other implement to release the catch on the cylinder body. Insert it into the hole as illustrated, and pull on the handle of the key - the cylinder should slide right out.
If it doesn't come out, try turning the key a little further or a little less as you push on the screwdriver.
If you can't turn the key in the cylinder, follow the instructions at this link: http://www.motorage.com/motorage/data/articlestandard/motorage/472002/38913/article.pdf
If you do drill out the cylinder, be extremely careful not to lose any parts from the broken cylinder, or the rest of the job will be either difficult or impossible. You want to keep the tumblers in order. Read ahead if you're not sure what that means, then come back to this step.
Step 6: Get Yourself Organized
An important note: you need a workspace with a light-coloured background! A lot of the parts are tiny, and are easily lost. I used a couple of notebooks as workspaces to keep parts easily visible. Two were necessary, because you need to keep old and new parts separate so they don't get confused and end up in the wrong place.
Organization is critical! If you lose a part, mix up pieces, or get confused, you'll end up with two worthless lock cylinders and a car that won't start!
Take the black snap-ring off each lock cylinder, and place them aside (remember, keep them organized!) I used a clamp and a flathead screwdriver, but snap ring pliers would have been much easier. Slide the inner cylinder from the outer, and place the outer cylinder aside with the snap ring.
Step 7: Get to the Tumblers Already!
On the inner lock cylinder, you will see a flat bar that sticks out, and about 90 degrees around, a thin piece of sheet metal covering the tumblers. The bar is what keeps the inner cylinder from rotating until the key has been inserted.
Remove the tumbler cover from each lock cylinder (old and new) one at a time. Do this by prying gently at one end of the cover. I would recommend starting with the old cylinder, in case you lose some of the springs that are under tension, and might come shooting out. Consider it a practice run (because you probably won't need them, but hang on to them just in case).
As the springs come out (either forcefully, or gently pulled out with an eyeglass screwdriver), place them aside. Order doesn't matter.
There is a piece right near the keyhole that may happen to slide out - if it does, set it aside and take out the spring that goes with it.
Finally, remove all the tumblers from the new cylinder, but leave the tumblers intact in the old cylinder. To remove the tumblers, insert a key to push them to the top, then use a slightly bent small paper clip to hook them and pull them out.
Step 8: Re-key Your New Lock
Starting with the tumbler nearest the tip of the key, remove one and only one tumbler, and examine it closely. To take it out, I inserted the key to raise the tumblers, hooked the paper clip under the part, and manipulated the bar on the side of the cylinder until it came free without forcing. The tumblers in my old lock are shaped like a lower-case y in that old "computer" font (see picture). There is a notch on the long side. Take a close look at the notch.
Now find a tumbler from the new lock with the notch in the same place. If the notches match, then you've found one that will work for that position. Put the matching tumbler from the new lock into the slot closest to the key tip in the new lock cylinder. Press it down with the paper clip tool. If it is very loose and in danger of slipping out, you might want to put some grease on it to "glue" it in place. Mine was tight, and there was no danger of the tumblers falling out on their own.
Continue working from the key tip to the key body, one tumbler at a time. Hopefully you will be lucky and most or all of the tumblers will have a matching one with the notch in the same place. If both your cylinders have the same type of tumbler, you can just swap them into the new part.
If necessary and if possible, you may try to make the old tumblers fit the new lock. I had to grind a small part off an old tumbler to fit the new lock - I was one tumbler short. Take your time, and grind slowly because the brass will disappear quickly! If you can't get the old tumbler to fit the new cylinder, it is an option to leave it out entirely. This will compromise the security of the lock, but you need a programmed key to run the car anyways! I've even read of people who removed all the tumblers and rely only on the PATS system. I don't recommend this, but it's an option.
Step 9: Close Up the New Tumblers
Carefully insert the original key into the new cylinder, and check if the bar on the side of the cylinder sinks in flush with the cylinder surface. If it does, it will allow the key to turn in the lock. If not, you've got a tumbler mis-matched to the key. When you remove the key, press each tumbler down with the paper clip and check that the bar sticks out from the side.
Once you've verified that it seems to work, you can begin to close the new cylinder. Dab thge bottom of each of the tiny springs in a small amount of grease, then insert it above each tumbler. The grease will hold it in place for you.
Check the tumbler cover to see if the corners are still at 90 degrees. When I pried mine open, the sides opened up a little. Use your pliers to gently bend the sides so they are square to the long part of the cover. A flush fit is important so your lock doesn't bind. Press the cover on with your fingers.
Now open your vise-grip pliers up wider than the lock cylinder. Close the jaws of the vice-grips loosely over each side of the cover, and turn the adjustment knob until the jaws touch the cover. Open the pliers, tighten the knob a quarter turn or so, then close the jaws again to gently press-fit the cover back on the cylinder. Be gentle here! If you damage the cover, you'll cause yourself more trouble. Take your time, and only press the cover on as tight as necessary.
Step 10: Reassemble Your Cylinder
Insert the key into the cylinder, then slide the cylinder into the outer cylinder that you removed back in step 6. You'll need to line up the bar on the side with the groove. Once it's in, you can remove the key and try to rotate the cylinder. If it won't turn without the key, and will turn with it, then you're almost done.
I ran into trouble here, and it took a little bit of puzzling to figure out. It turned out that the tumbler cover was not completely flush with the side of the cylinder, and it would catch in the groove in the outer cylinder. The symptom was that the key would turn, but then not turn back. If that happens, check the cover.
Once you've got it working, you can replace the snap ring.
Step 11: Put Your New Lock Into the Steering Column
Be careful of the black ring around the lock cylinder - that's part of the PATS system, and if you bust that, it's probably going to be troublesome and expensive.
Now that the lock is in place, test it out. Don't worry - you won't start the car, because hopefully your battery is still disconnected! Cycle the key a few times to make sure that everything is as it should be.
Once your new lock is in place and working properly, you can go about putting everything back the same way it came out. If you disconnected any electrical connectors, put 'em back in now.
One thing to note: as you put the upper shroud on the steering column, there is a rubber skirt with tabs that press into the underside of the instrument cluster. Take your time, and press them in place to ensure a good fit.
You're done! Hooray! Hook up your battery, and enjoy the knowledge that your time is worth quite a bit of money still in your pocket.
Step 12: Put Keys Away, Drink Beer.
I hope that this helps out some folks who have a bit of time, a bit of determination, and a love of frugality. Please comment if you've had this problem, or if I've helped you out in any way.
Good luck, everyone!