Hi. I’m new to the site having just bought my first MX5 NC.
I bought a new key fob body and swapped the blade and electronics from the old key fob. The doors and boot unlock no problem but when I try to start the car the immobiliser drops in. Any suggestions on what the problem is and how to solve it?
Did you miss transferring the ID chip that the car needs to detect to allow starting?
I’m not sure what does it look like?
Someone with a NC key fob will need to answer that, the last one I saw was in a Citroen key…
Ok thanks for the help
Have a look at post 4 on the attached thread.
Our friends in America say it can be done at home if you have both keys.
Good luck and would be interested if it works. 
https://forum.miata.net/vb/showthread.php?t=641729
Which is this.
https://forum.miata.net/vb/showthread.php?t=435772
The way I read it is as follows.
Follow the first section to program the “Actual Metal Key”
The second section is to program the “4 Part Fob” i.e the Lock/Unlock/Boot Unlock/Cancel Alarm.
Again good Luck! 
The chip is next to the blade in the top part of the key fob. you will need to open it,then it’s clear to see.If unsure hold your old top key part next to the ignition and try to see if it works while turning the new fob
You’ve got me thinking now. 
I just checked mine (and not going to pull it to pieces).
But separated the key from the fob.
It confirms to me that the “Immobiliser” is on the actual key part and not in the fob as I tried it.
But like GeoffN1 I can see nothing obvious (the chip) within the key part that you can take off.
I see the chip in the circuit board within the fob and get that.
So if you have a picture of the bit you mean that would help us all perhaps for now and the future.
Many thanks.
Yes, the immobiliser chip is in the blade part of the key. It’s a little black block.
Ahh so like the “pronged bit” going south from the key and embedded in the actual plastic?
Cheers Robbie.
Just to add, it will probably be ‘glued’ in. It can be removed with due care.
I would advise swapping that fob case part over complete with chip. Not sure what glue they use to fit the chips to the case in the MK3 but probably stronger than the chip!
Cheers guys make sense to me now and hopefully the originator gets it sorted
Here is a pic of a blank key and its plastic housing without an immobiliser chip. I put a scrap of paper in the slot where it would live to make this easier to see.
The screw holding it all together is hidden under the Mazda badge.
I needed the other half where the circuit board lives simply because the button sheet had lost its integrity.
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As they say, “A picture paints a thousand words!”.
It’s not where I thought it was.
Thank you and brilliant.
I now have one working key for my wife’s car as I sought to remove the glued in transponder chip…
Thank you to all for the support on this issue . After a root through the bin I found the key blade head section with the chip in it. After having a go at digging it out I made a decision to keep that part of the fob and to attach it to the side without the chip in. The spring was a challenge but I now have 2 working keys again.
Thanks all
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Good news