I've had to replace the fob casing for my MX 5 and moved the circuit board and the key blade to a new case (ebay) but now the immobilizer indicator on the dash blinks and the car won't start. The fob locks and unlocks the car.

Is the transponder on the circuit board or is it embedded in the old fob case somewhere.

I also have just the one key :-/

 

What year MX-5? Key is embedded and you stated you replaced the blade, are you talking about the key itself?

 

In case anyone has the same problem in the future...

the transponder is the top section of the fob (the bit that holds the key blade) and is a little object shaped like a grain of rice tucked into a small cavity.

To access that bit of the key fob you will need to remove the Mazda logo to expose a small screw.