Photos: 1) New key from aliexpress, 2) An old key showing some wear n tear.
This is a very rough guide to replacing the faulty case (fob, shell) of your 'flip' car key. This guide does not include replacement of the internal transponder and circuit board. If the former is you, then you're in luck as the process is relatively straightforward - you can do it!
I couldn't find anywhere that sells just the button pad, to just replace that. If you do not want to replace the entire key case, and only want to replace the button pad, try the link at the bottom of this page. You could order the key case, and cut out the button pad from the new key. I preferred to replace the whole case.
I was quoted between NZD $350 - $450 to purchase a replacement key, by Hyundai NZ and key cutting shops. They would not repair it.
I did this repair for $7.00 total, excluding my time (but the satisfaction was worth it). I already had several of the standard/basic tools that I needed. I'm not sure on the rules of 'Instructables', but if you want me to do this for you, get in touch.
Warning: this guide is only valid if you still have the (now defunct) key(s), as you'll need the innards/electronics to swap into the new case. If you've lost the keys, this guide is not for you (you may have no option but to go to the car manufacturer/agent). Also, if you've only got one key, and you're REALLY not confident, perhaps paying $350 is a better risk than stuffing up the only key you've got and paying a lot more for a whole new set.
My car is a 2012 Hyundai i40. The key case I purchased, from aliexpress (see search criteria below), was a perfect match.
You may not know what the inside of your key will look like to find the exact match. If you're reading this, you've probably run out of other options, so I suggest you open the case of your busted key, assuming it's a common car brand. If it all goes pear shaped, you'll at least be able to take the electronics to the $350 option and swallow your pride (perhaps check with them first). Or just go online and find the replacement case you think will be the exact match, spend a few dollars and order it, and if it looks the same on the outside, there's a good chance it'll be ok on the inside.
The Car Key Manprovided the inspiration that I could do it. I recommend his video, probably before continuing with this.
I purchased the key case from aliexpress: search "KEYYOU 4 Buttons Flip Folding Remote Key Shell Fob Case For HYUNDAI Mistra Santa Fe Sonata Tucson Accent I30 I40 I45".
Step 1: Open the Old Case
A) take off the rear battery cover. This should just pop off with some thumb pressure.
B) remove the battery.
C) There are two possible ways to open your old case.
1) the case sides have been clicked together and will simply pop open with some pressure applied to the seam around the middle, as shown by twisting with a chisel.
2) the case is a single solid piece. The 'seam' is just for show and wont 'crack' open.
This is the time consuming part.
Using a soldering iron*, melt the plastic around the fake seam. Work down through the plastic slowly, so as not to damage the circuit board. As you get around the edge of the case and it starts to come apart, be careful to catch anything that falls/springs out.
*If you don't have a soldering iron handy ($10 for a cheap one), you could similarly use a hack saw.
The last photo shows the end result. The boxes highlight the (top) transponder cover, (lower) a circuit/button cover from the circuit board that fell off when opening. Easily re-affixed.
Step 2: Swapping Out the Key Shaft
You could take the new key shaft into a cutter and they could probably cut you a new one, for a fee. And you wouldnt need to do this step. But its very easy - with one important check to make - see that the pin slots line up as in the last photo on this page.
The key shaft has a pin which you need to knock out. I used the spike shown in photo 1, and then pulled the pin the rest of the way out using pliers (careful not to crush the pin).
You have to do the same with the new key shaft. This was a bit stickier, and I used the small screwdriver to push it out fully.
As in the last photo on this page, the slots aligned perfectly. If they don't, you won't be able to reinsert the pin, so you may have to get a small file and make a new slot. See the link on the first page (Car Key Man) who covers this.
Step 3: Remove the Transponder
The transponder will be covered - in my case by a white plastic cover.
Screwdriver to lift it. I used a stanley knife/sharp blade to gently pry loose the transponder from the case. Sometimes they superglue them in there, but mine popped out after some persuasion.
If it's not already obvious, the key won't work without the transponder. Don't lose it.
Step 4: Assembling the New Key Pt.1 - Circuit and Transponder
Open the new key case: remove battery cover, ensure screw is not in the back.
This should pop open using a blade twisting along the seam (see pic on page 2?).
Place the transponder into the new case recess. I used bluetak to wedge it in (it'll rattle around if you don't glue/wedge it in there).
Pics show how this should look.
Step 5: Assembly Pt.2 - Key Chain Part and Key Shaft
Photos should be obvious (albeit blurry)
1. The key chain hook part slides into the case. I had to push hard to get it to click.
2. Place the spring pin into the slot in the case.
3. Bit tricky - wind the key shaft in the direction of the red arrow. A couple of complete turns/winds should be plenty. The blue arrow is to show the direction the key blade needs to flick up when it's working.
4. Push the wound shaft int the case proper.
5. Another view of it.
Step 6: Assembly Pt.3 (final) - Body
1. Bit tricky - get the circuit board as 'centred' as possible and bring the two halves together. Obviously, don't crush them together if there's something not right (have another go), but you may need to push hard to get the halves to click.
2. Screw on the rear
3. Battery (duh)
4. Test (OMG, it worked :)