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I have heard that it is possible to unlock a car using the remote key-fob over a cell phone network.

Scenario: Adam locks his keys in his car. He uses his cell phone to call his wife, Betty, who is at home with a cell phone. Adam holds his cell phone close to the car while Betty holds the spare remote close to her cell phone and pushes the UNLOCK button.

Will this unlock the car door?

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    Where have you heard this?
    – Nicole
    Apr 5, 2011 at 15:18
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    A few years ago a colleague managed to persuade me that this worked. Ten minutes later I was standing outside in the office car park while he phoned me and triggered my car's key-fob. Conclusions of this experiment: 1) This does not work. 2) If you try it in the office car park, everyone you work with have confirmation that you are, in fact, an idiot. My advice: if you feel moved to answer this one empirically, find somewhere remote. :-)
    – Simon Whitaker
    Apr 5, 2011 at 21:25

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According to HowStuffWorks' page on How Remote Entry Works:

The fob that you carry on your keychain or use to open the garage door is actually a small radio transmitter. When you push a button on the fob, you turn on the transmitter and it sends a code to the receiver (either in the car or in the garage). Inside the car or garage is a radio receiver tuned to the frequency that the transmitter is using (300 or 400 MHz is typical for modern systems).

And, from Telephone on Wikipedia:

The telephone [...] is a telecommunications device that transmits and receives sound, most commonly the human voice.

And again, from HowStuffWorks on Cell Phones:

To start with, one of the most interesting things about a cell phone is that it is actually a radio -- an extremely sophisticated radio, but a radio nonetheless.

But, nevertheless, a cell phone intentionally transmits on frequencies specifically assigned for mobile phone use.

So, even though both cell phones and keyless entry remotes are radios, there are two main reasons why this is not possible:

  1. From the transmission end of the call, only audio is transmitted onto the phone network (via microphone into an analog signal and then via electronic components to a digital signal, if a digital cell phone is used). Radio signals are light, and are not audible.

  2. That audio is transmitted to your cell phone via cell phone frequencies. Your cell phone speaker only emits sound. As mentioned above, though your cell phone is a radio transmitter, and does emit radio signals, it only emits radio signals on cell phone frequencies for the purpose of transmitting audio. It has no functionality for transmitting on any other frequency (like the band used by your remote), let alone for any other purpose. (See the Wikipedia page on Frequency allocation for information about regulation of radio frequencies)

Subjective and speculative footnote: Looking at the technologies being used by cell phones and key fobs, it seems reasonable to believe that a cell phone could be built for this purpose. However, it certainly wouldn't be a "mystery" function, since the device would have to have explicit approval from the FCC (in the US) to function on that band (check your key fob for its FCC approval ID). I can't imagine any cell phone maker building this functionality, getting it approved, and then not advertising it.

Also, it's probably unlikely that it ever will be made due to more advanced, reliable, and propietary methods of unlocking cars (like GM's OnStar) are already in development or in use.

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  • Thanks. I didn't had a reference yet to post an own answer. It should also be noted that modern cell phone can have a backup infrared connection, so that they also work in environments with a lot of radio noise. However, cell phone do not transmit infrared light either.
    – Martin Scharrer
    Apr 5, 2011 at 15:36
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    The speaker in a mobile would also be in-capable of reproducing radio waves, to further put this to bed.
    – Tomas
    Apr 5, 2011 at 15:38
  • The first celular telephones used a completely analog system, the voice channels of which could have, in theory, transmitted a key fob radio signal. The problem is that those cell phones (at least in the USA) ran in the 800MHz spectrum (see link for reference), which is roughly twice the frequency of modern key fobs (I am not sure what frequency the fobs used in the 1980s though). This would have likely required hardware modifications to the phones, and it would of course not work on modern, digital cellular networks.
    – ESultanik
    Apr 5, 2011 at 19:29
  • @ESultanik I agree that in theory a phone could be built to do just that. But the question is, not whether it is analog or digital, but how would the cell phone on the transmitting end "hear" the signal? A microphone doesn't pick up radio waves.
    – Nicole
    Apr 5, 2011 at 19:56
  • @Renesis I agree completely. My point is that if the phone itself is analog, then it could potentially "hear" the key fob signal by virtue of the fact that it is already listening for raw, analog radio signals over the cell network. If the cell network were on the same frequency as the key fob, and if there were some sort of odd feedback loop or malfunction in the phone, it is possible that it could pick up the fob's transmission and re-transmit it over the cell network. This is of course hypothetical and very unlikely.
    – ESultanik
    Apr 5, 2011 at 20:32
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This was tested and busted by Mythbusters.

It is also listed as urban legend / email hoax by About.com.

The signals emitted by the car remote are mainly radio signals with a frequency of 315MHz in the US and Japan and 433.92MHz (ISM band) in Europe as explained in Requirements of Remote Keyless Entry (RKE) Systems. An alternative are infrared remotes which are not affected by radio noise generated by some machinery e.g. in industry, but require to aim to the car.

Mobile (aka cell) phones are only designed to transmit human voice or digital data like text messages. The frequency bandwidth of the transmitted audio signal is in the range from 3.1kHz to 4kHz (8kHz sampling rate, which result in (almost) half the bandwidth following the Nyquist–Shannon sampling theorem).

Mobile phones use a normal microphone and loud speaker and an analog-to-digital as well as a digital-to-analogue converter to record and play the transmitted audio signals.

It is easily understood that a microphone can not record either radio waves nor infrared (light) signals. Also the loud speaker of a mobile phone is not able to produce radio waves or infrared signals.

As shown by the above urban-legends link, this idea popped up as an email and following the technical explanation shown here can be safely taken as an hoax.

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  • IF a remote sent IR pulses, a phones camera could pick them up (point an IR remote at most digital cameras and you can see it flash). you could write an app that picked them up, and transmitted the code across to another phone. If that phone had an IR emitter, it COULD re-generate the code.
    – fred
    Apr 5, 2011 at 17:32
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    I'm always interested in how these urban legends get started. I believe this one got started when hackers demonstrated that particular models of cars that used infrared remotes could be hacked into using smartphones (such as Palm or Windows) that have programmable infrared transmitters on them (which were used for wireless syncing and business card exchange). Clearly the story mutated wildly since then, but the infrared attack is real. Some Mercedes in particular have this, the receiver for the infrared signal is mounted behind the side mirror.
    – Tim Farley
    Apr 6, 2011 at 13:46
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    @Tim Farley: Or at least, used to be real at some point in the 1990s - most modern remote unlocking systems use a cryptographic challenge-response system, transmitting over the radio part of RF spectrum (amongst other things, to thwart these - essentially, message replay - attacks).
    – Piskvor left the building
    Mar 29, 2012 at 17:29
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    @fred To my knowledge, different frequencies are indecipherable on a camera.
    – Allison
    Jun 25, 2013 at 21:38
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    @BenVoigt Well, I've learned something in the intervening decade :) Modern CMOS cameras don't actually sense the frequency of the light, rather they observe they have color filtered (i.e. R, G, and B!) light sensitive gates which accumulate charge when exposed. So, yes and no. They don't sense frequency directly but they sense how much red, how much green, and how much blue is emitted because human eyes are trichromatic and can't tell the difference between raw frequency and a linear combination of three colors!
    – Allison
    Jun 6, 2022 at 6:38
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Yes, if the car is designed that way

I'm cheating slightly as I'm answering the headline not the detail of the claim. But there are devices that allow phones to unlock a vehicle. They involve fitting the car with a black box that connects to the mobile phone network and allows location and unlocking functions controlled from the phone.

I first saw this in the London Evening Standard who reported that the system was developed by Zipcar for the convenience of customers using their rent-by-the-hour cars.

This is likely to become a lot more common as smartphones become ubiquitous.

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