I'm not sure if this question is ok to ask here but I'm hopeless in finding sufficient hardware for it to be done.

I have two pairs of keys to the car and one of them stopped working so I want to repair it or at least check what's wrong with it. After measuring it with a multimeter, I came to the conclusion that it's working properly but it still won't open the car.

So I would like to check if the signal is even sent, for that I need some kind of radio receiver (at least I guess that is what I need). I would like to buy a receiver that can be plugged into PC by USB where I would be able to check the frequency and data sent for future comparison with working key.

A link or some key phrases which I need to look for would be great if anyone does know something like that to buy.

  • \$\begingroup\$ I'm afraid, asking for specific product recommendations are off-topic here. \$\endgroup\$ Dec 9, 2021 at 12:39
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    \$\begingroup\$ I don't think this is a specific product recommendation, as OP is asking for key phrases to kickstart his research into this. I would start with searching the web for a 'software defined radio'. Have fun. \$\endgroup\$ Dec 9, 2021 at 12:40
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    \$\begingroup\$ As @LukeHappyValley said, I'm not looking for specific product. I just didn't know what to look for to work with my problem. When you type in google 'radio receiver usb' you mostly get some wifi modules or something like that. Thanks Luke, I already find a direction where to go thanks to you :) \$\endgroup\$
    – Morph21
    Dec 9, 2021 at 12:58
  • \$\begingroup\$ You could use a simple RTL-SDR receiver with SDRsharp. Just to verify if the key outputs any RF at a given frequency, you'll do in minutes. Actually decoding the data sent would be a bit more work, but certainly doable. You can easily buy the HW needed for a few tens of $, \$\endgroup\$
    – Klas-Kenny
    Dec 9, 2021 at 13:07
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    \$\begingroup\$ Usually one just buys a new key from the manufacturer and it will cost the same as the equipment you're trying to get \$\endgroup\$
    – citizen
    Dec 9, 2021 at 13:31

1 Answer 1


I'd expect these to operate in one of the license-free bands at 433 MHz or 868 MHz, that's the easy part.

The protocol of any modern car key will be a challenge-response system with a very strict time limit, so the key will send a generic identifier, the car will send a random number, and the key will perform a cryptographic operation on that number and send back the result.

The car will check both the result and the response time, to avoid range-extension attacks where people use a repeater to bring a key into range when the owner doesn't expect that.

If you analyze the protocol without the car in range, you will see a simple transmission of a serial number, nothing else. There is a good chance that both keys will use the same.

With the car present, you get two additional transmissions of effectively random data, comparing these will not give you any additional insight.

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    \$\begingroup\$ Car keys are usually transmit only. The common algorithm (Keyloq) doesn't do challange/response. \$\endgroup\$
    – Lior Bilia
    Dec 9, 2021 at 21:04

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