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I have a recently bought new Kia that came key fob which is composed of two parts. The actual electronic device with the buttons, and this part slides on a cut key which has the keyring hole attached to it. The two are usually very well attached.

Somehow, the two got separated and the device part of the fob fell off. I am 99% sure it is somewhere in the house, but just can't find it.

Given that it's a transmitting radio device, can I use this to find it?

  1. Do these things constantly transmit? If yes, at which frequency? If no, how can I make it transmit?
  2. Which tool would I be able to use to locate it?
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    Welcome to Motor Vehicle Maintenance & Repair! To my knowledge, the fob only transmits when a button is pressed. I really doubt there is any way to find it as you are suggesting as it just doesn't work that way. The car receives a signal from the fob and that's about it. It doesn't send anything back ... at least not stock ones. I've seen some aftermarket ones (Viper) which do, but that's from many moons ago and I don't even know if they are in business anymore.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 24, 2022 at 0:51
  • @Pᴀᴜʟsᴛᴇʀ2 it's a button-less fob. All I have to do is have it in my pocket, press a button on the car, and it unlocks. The question is whether the fob constantly transmits, or it only replies to the car when I press the button on the car door.
    – Gimelist
    Commented May 24, 2022 at 2:10
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    DOH! You're right! I had completely forgotten about keyless entry and such. Reading a little bit on it, it seems there are mixed ideas (nothing concrete) about how these types of fobs work. Some suggest they're "always transmitting", while others say the car always interrogates, which wakes the fob up when in range. Considering the size of a fob's battery, to me it would not make sense for it to be always on, as this would deplete a CR2032 in no time. The interrogation/response seems like a more viable solution ... but ... I wouldn't know for sure.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 24, 2022 at 12:01

2 Answers 2

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The key fob has an RFID chip in it, which responds to a wake-up signal from the car by sending a return signal (probably in the UHF band). If you had a device that can transmit that wake-up signal, and another (or the same) device that can detect the return signal, then you could indeed find your fob this way. You would send the wake-up signal repeatedly as you search, and when you detect the response you would follow the signal to the fob.

That's the answer to the question: "Is it possible". As to whether it's practical, almost certainly not. There's a fair amount of specialized tech that you would need to obtain, and a fair amount of specialized knowledge required to use it effectively as well.

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My wife has a 2018 Honda CRV and the remote fob is always transmitting without pressing any of the key fob buttons. As I walk up to the car the doors will automatically unlock, so it is always transmitting an RF signal. The automatic door unlock is a user option I setup in the vehicle settings. If your Kia does the same you could rent or buy an anti-spy/bug detector. These devices can pickup RF signals from 1 MHz-8 GHz or greater. The purchase cost is ~ $50.

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  • I actually have the other key (we have two). Using one of those devices I can see whether the key that I still have transmits, and then if it does I can use that to find the missing one?
    – Gimelist
    Commented May 24, 2022 at 2:13
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    I read somewhere that US model cars transmit the RF signal at 315 MHz and foreign cars transmit at 433.92 MHz. You could also call Kia and ask if they would provide the RF frequency for your make and model. It should not be a security concern because they use rolling security codes.
    – MJH
    Commented May 24, 2022 at 2:50
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    Are you sure it is always transmitting, or is it just always listening for the transmitter in the car and it responds when it hears the car? I don’t know, it just seems a more power efficient way of doing it to save key fob battery life.
    – HandyHowie
    Commented May 24, 2022 at 5:30
  • @Gimelist your idea is a good one and should work well. Good luck!
    – MJH
    Commented May 24, 2022 at 12:02
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    @MJH Continually emitting RF would, as Paulster2 remarked in comments to the original question, use a lot of current and quickly deplete the small battery within the fob. More accurately, the fob's battery powers a small, always-on receiver in the fob; when this receiver detects the RF signal transmitted by the car to which the fob is matched, the fob then transmits a coded response; the car receives the response, and seeing that it's correct, then unlocks the vehicle.
    – DavidRecallsMonica
    Commented Jun 23, 2022 at 14:13

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