- Problem Solved
- My model of MX-5 is: NC 2007 1800cc__
- I’m based near: Milton Keynes__
- I’m looking for technical help or recommendations on: Ive just replaced the battery and now the car won’t start. It seems the immobiliser is stopping me starting the car. All the lights come on in the dash and the key light flashes when I turn the ignition key. I cleaned the battery connections as well.__
Check the fuses (under bonnet) particularly the one for protection of the ignition and maybe the room fuse.
Anything else not working, like interior lights, radio etc, does it turn over?
To replace the battery and reset the steering on a Mazda MX-5, follow these steps:
- Turn the ignition switch to the ON position.
- Turn the steering clockwise fully, then turn it counterclockwise fully.
- Make sure the DSC OFF indicator goes off.
- Turn the ignition switch to the OFF position again.
- Make sure the TCS/DSC indicator light goes off.
Check the battery charge level. It may have sat on a shelf for months.
Thanks for the fast reply, all the ignition lights come on plus interior lights radio etc. I don’t think its a fuse.
Battery fully charged overnight before fitting it.
I don’t think the car has DSC, it’s a 2007. Ive only had it about a week.
No DSC on the 1.8’s.
I think there’s a procedure to relearn the key, not sure myself of this, have a search or wait until someone pitches up with an answer.
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How recent is the key battery? Worth trying a fresh, good quality battery in there - just to rule that aspect out.
Brand new charged battery
I’ve just read your post a couple of times.
Are you saying the battery won’t engage the starter motor and turn the engine?
Just clicks perhaps and does nothing?
Or it does crank the engine, but not start?
If it is turning the engine over firstly make sure you haven’t got your foot on the accelerator pedal as if you have it won’t start.
The fault diagnosis is different for the two scenarios.
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The 2 keys I got with the car were in bad shape, so I purchased 2 new cases from Ebay. I changed over the 2 circuit boards and fitted new batteries. The keys came as blanks which needed to be cut, which I had done locally. All worked fine until I changed the car battery, this seems to have reset something in the immobiliser as the keys now wouldn’t start the car. I fitted the fob electronics back in the old case and the car starts fine. Further investigation which entailed taking the top (key) half of the remote apart and the original Mazda key has an additional small metallic like block inside. I can only assume this is something the car is reading to disable the immobiliser. Getting the fob back together is very tricky, 2 springs and a lot of fiddling, but I have one back together with a new bottom half.
It’s very important to remove and replace the “chip” from the key head to the new key/body.
I “believe” this is correct.
The chip 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.
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Yes you are correct thanks, just found this out.
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See you have edited as problem solved.
Is it the key chips and does it now start okay? 
Yes that was the problem.
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I worked in IT for 30+ years, and you’d be surprised how many times we only get half a story. What started out as a battery replacement was something totally different and not offered details for diagnostics. 
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I don’t understand, what other diagnostics can I offer?
Mentioned the keys in the initial post asking for help.
Anyways you are sorted that’s the main thing, enjoy the car they are mostly trouble free a few niggles can crop up though.
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You failed to mention that you’d messed about with the fobs, so folks came in to this trying to figure out why the car wouldn’t start after replacing the battery