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(2018 cx3) One fob works great. Other doesn't work "electronically". I've replaced battery 2x. The buttons will not work to lock/unlock the car when outside of car and the tiny red light doesn't light up on fob. I can open car door ONLY with physical hard cut-key. I can turn on car by holding the keyfob to the ignition button, waiting until green and car will start. However, if I have key fob on the seat (not held up to ignition button), the car doesn't recognize it and the red lock light comes on. Its as if the car recognizes that the fob is broken so still allows me to turn it on by holding the dead fob up to the ignition button. Otherwise, the electric chip/board inside the fob isn't "talking" to the receiver in the car for "key-less" use. Does it need to be reprogrammed or is it shot? If it is reprogrammed, will the car assume I'm reprogramming both fobs? I don't want to lose connection with my working fob. Worst case scenario, I have one fully-functioning fob and other that is manually working by not electronically working.
 

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When you hold the fob to the start button, the start button checks an RFID chip in the fob by emitting radio waves.
A powerless (dead battery) or damaged fob, but with an undamaged RFID chip, still enables you to start the car.

Maybe your fob has stopped working, or there is a problem with the battery.
 

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If im skiming this right, you tested both keyfobs and one consistently works while the other has zero connection even if switching batteries. The chip is working if you do the manual method with the rfid option so this makes me think its a hardware issue. Could be something as simple as battery connector not making contact or a loose solderjoint. Unfortunatly this can happen when keys are dropped especially when sensitive electronics and why I usually tell people to get the little rubber(not hard plastic) covers for the mazda keyfobs.



Try seeing if its a battery terminal issue. Bend up the metal contact pin on the bottom of the rob citcuitry so its more likely to connect. Swap with batteries off of the working one just to reconfirm its not bad luck with doa batteries. Open the keyfob if nothing else is working and look for anything out of the ordinary and see where connections come and go and if theres anything disconnected. I feel like if you get it reprogrammed it wont be doing anything if theres a hardware issue. At that point if all else fails it would be better to buy a new fob online since dealers overcharge horribly and get that programmed which will deactivate the old keys (should since its a security issue same as if one fobb gets stolen you want nothing to work and reprogram everything new). The manual key should be swappable since sleeve is the same and latches in.

This is assuming you are the original owner or got the car with both keys working at some point. I wouldnt be surprised if people would use random unprogrammed ebay keyfobs after breaking one and just throw in the manual key into the sleeve and act like its '2 keys' when selling/trading it in.
 
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