There's been some security-related questions being asked here and on sister sites concerning keyless ignition automobiles and how to defend against some security risks. This brings up a related question.

Why do cars keep running if the car can no longer detect the keyfob?

  • 16
    To avoid fiery death when the fob battery dies on the highway?
    – JPhi1618
    Commented Mar 17, 2016 at 17:58
  • 1
    @Ellesedil, its a safety feature everywhere you can drive not just highway or intersections. I wouldn't want my keyfob battery to die while moving through a mall parking around Xmas, those places can be dangerous Commented Mar 17, 2016 at 18:28
  • 6
    Interestingly enough, there have also been lawsuits because the cars don't turn off. Must be tough being a car designer...
    – JPhi1618
    Commented Mar 17, 2016 at 18:46
  • 1
    Here I wrote about an incident where the car did stop on the middle of a highway, because a child had thrown the key out the window.
    – sweber
    Commented Mar 17, 2016 at 19:53
  • 1
    @HandyHowie - regardless of technology used, it's all RF. If your child throws it out the window or you accidentally drop it in your metal coffee mug, you don't want the car to shut off unexpectedly while you're driving. The times you do want the car to shut off without the keyfob (i.e. if someone carjacks your car) are much rarer than the cases when you don't (and will just train carjackers to demand the keyfob or take it from you forcibly when they steal your car).
    – Johnny
    Commented Mar 18, 2016 at 16:04

5 Answers 5


As JPhi1618 mentioned, this functionality centers around CFR 571.114, AKA FMVSS 114. Specifically section 5.2.1 which states:

Except as specified in S5.2.3, the starting system required by S5.1 must prevent key removal when tested according to the procedures in S6, unless the transmission or gear selection control is locked in “park” or becomes locked in “park” as a direct result of key removal.

An important point to understand is that car manufacturers consider the electronic code transmitted from the key-fob to the starting system the digital equivalent of a physical key, so the "removal" of the "key" occurs when the driver presses the button to shut off the car and the code is cleared from the starting system.

This would make it trivially easy to violate the regulation if the car cleared the start code as soon as the key fob was too far away, as it would allow a situation where the "key" can be "removed" before the car is put into park. So to skirt around that regulation, the car simply remains running so the car makers can argue that the "key" cannot be "removed" before the car is put into park. As an aside here, you could argue that it still violates the regulation, as many cars with push button start/stop will stop without the transmission being set to the "park" position when the start/stop button is pressed.

It looks like the last time CFR 571.114 was updated was in 2010, and it hasn't really kept up with this technology. There's a lot of grey area, lawsuits, and it'll probably be a few years before the law and technology fully mature here.

Edited to address @user1663987's comment: If the car engine stopped when the key-fob is moved too far away, then the car mfg would have to implement a system that physically prevents the driver from removing the key-fob from the car unless the system is in park to stay in compliance with the regulation. This would pretty much defeat the "key-less" point, as the best way to "lock" the key-fob in the car while it's running would be to just have a key on it and use a traditional key-based ignition. So in a sense, this behavior is a "desired" interpretation of the regs because it the car mfg doesn't have to kill a desirable selling point for their vehicle.

There is a safety aspect that can be considered, in the event that the key-fob is unexpectedly removed from the vehicle while driving (e.g.: child tosses it out the window), however this isn't something that's currently addressed in the regulations. It's possible that auto makers have looked into this already and found that a vehicle with a key-less ignition that continues running regardless of the proximity of the key-fob is safe, though reports of people leaving their vehicles running by mistake would indicate otherwise. At this point I've wandered into the realm of speculation, so I'll stop here.

  • 2
    That's quality. Thanks for contributing to the site. Good stuff. Welcome and Cheers! Commented Mar 18, 2016 at 1:01
  • Actually this is an excellent and appropriate answer, well done! And welcome to the site!
    – cdunn
    Commented Mar 18, 2016 at 14:09
  • Putting this as a comment would have been a disservice to the information you provided. Great answer.
    – Ellesedil
    Commented Mar 18, 2016 at 16:57
  • this answer basically says "so mfg can honor the words of the law while ignoring the spirit of it". it says functionally why they (probably) still run, but not why that might be a desired outcome of that interpretation of the law.
    – simpleuser
    Commented Mar 18, 2016 at 17:58

I looked into this, and it turns out that almost all aspects of starting or turning off a car are governed by a federal requirements document called FMVSS 114. Things that are in there:

  • Steering wheel lock
  • That little button you have to squeeze on the gear shifter to take it out of park
  • Can't remove key unless you are in park
  • Vehicle can't roll when key is removed (because it's in park)
  • Your keyless ignition car beeps the horn when you get out but the engine is still running
  • ETC...

I can't find the most recent version of this document, but recent keyless ignition recalls and lawsuits have focused on this document.

Therefore, I say the reason this happens is because the government has mandated it to happen. I wish I could provide black and white proof. If anyone else finds a clear source post it in an answer or let me edit it into mine.

I found what looks like an older version of a compliance testing document, so that's something.

  • 1
    "Your keyless ignition car beeps the horn when you get out but the engine is still running" Mine doesn't do that.
    – JAB
    Commented Mar 17, 2016 at 19:50
  • It should give you some kind of warning. If it doesn't, maybe you should expect a recall? I'm also not sure if all that litigation/codification is completely done, but there are a lot of articles about carbon monoxide poisoning and cars that don't warn you when you get out and it's still running.
    – JPhi1618
    Commented Mar 17, 2016 at 19:58
  • That little button you have to squeeze on the gear shifter to take it out of park. Not for a Prius :) Commented Mar 18, 2016 at 9:54
  • 1
    The USA is not the world.
    – TRiG
    Commented Mar 18, 2016 at 11:28
  • 2
    @TRiG, Of course not, but since vehicles from so many other countries are sold in the US, the regulations here are very important worldwide. Other countries probably have similar standards documents, but I don't know enough to search for all that.
    – JPhi1618
    Commented Mar 18, 2016 at 13:23

There may be other reasons but without a doubt a big part of it is because it's impossible to predict under what conditions the car no longer detects the key. And as stated in some of the comments, that could be while driving down the highway, or on the street at just the wrong moment.

Hope that helps!

  • 3
    Truth. Car systems need to be designed in a way that expects something to fail and still handle it without crashing.
    – JPhi1618
    Commented Mar 17, 2016 at 18:02

It is pure and simple a safety thing.
The American law given above doesn't spell it out explicitly, but that is the goal of the given regulations - prevent accidental shutoffs while the vehicle is in motion so as to avoid potentially fatal accidents.

In Germany, the law is much the same though from what I've heard the phrasing is more explicit - the cars aren't allowed to shutoff unless the operator shuts it off in order to avoid accidents. That is to say, a car isn't allowed to automatically shut itself off.

The reason is quite simple:

The car doesn't know when it is safe to shutoff. As others have noted, it would be a bad thing to shutdown the motor on a highway.

The same reasoning applies to the keyless ignition system. If you shut off the car when the keyless fob goes out of range, then there's no telling where the vehicle is. So, the car must keep running until the operator shuts it off.

This is what makes the RSA attack on keyless systems possible. The thief uses a radio relay to make the car think the fob is close by, then starts the car and drives off. Once it is running, he can drive it anywhere as long as he doesn't stop the motor.

Anti-theft systems here also have to follow this law. If the thief can manage to start your car, then the law says he's got to be able to drive off with it and not have the motor shutoff while the car is being driven. This is not to help the thief, it is to protect other drivers when the thief gets out on the highway.

The anti-theft systems thing was bandied around here back in the 1990's. There were imported systems from the US that would shutoff the fuel pump if the car was hotwired - this is the same as having the motor shutoff. These imported anti-theft systems were illegal here.

  • 1
    Just to back up your line on anti-theft systems, I actually worked on Aston Martin's remote immobiliser anti-theft system. As you say, we were not allowed to immobilise the vehicle until it came to a complete stop.
    – Graham
    Commented Mar 18, 2016 at 17:30

I have heard that there were reports of cars rolling away due to drivers not turning the engine off with the car still in drive or reverse.Older ignition systems like in most cars on the roads today require the car to be placed in Park before removing the key.

Newer car systems with fobs are supposed to alert when the engine is still on.

  • 3
    See, the problem is that the OP asked a very specific question about one part/feature of a keyless entry system, and you've posted two links to sources that explain how the whole system works without really giving a direct answer. The OP is also asking about proximity keys - not remotes that you have to press a button on.
    – JPhi1618
    Commented Mar 17, 2016 at 18:37
  • @JPhi1618 How should I have answered the question or found an appropriate answer? Is there anyway I should have distinguished if my research was applicable for the question?
    – mr_tuner
    Commented Mar 17, 2016 at 21:52
  • I think you should know enough about the subject or be confident enough in your answer to write most of it yourself. I think that linking to external sites should be used sparingly and only when really necessary. Also since we try to get questions to focus on a single point, make sure your answer only address just what they asked for. If it's not clear what they are asking, post a comment asking them to be more specific. Thanks for your participation - look at other answers from high-rep users to see patterns in answer style.
    – JPhi1618
    Commented Mar 17, 2016 at 22:00
  • That you very much @JPhi1618 for your feedback. I appreciate it. Thank you for your time.
    – mr_tuner
    Commented Mar 17, 2016 at 22:03
  • Also check out the formatting help page. You can use it to make links pretty, make bulleted lists, format quotes and tables, etc. Formatting and summarizing sources might seem like a waste of time, but the point on here is for people to read the information as easily as possible. If it takes you an extra 2 minutes to format, and that saves a reader 5 seconds when trying to get an answer, that was worth it.
    – JPhi1618
    Commented Mar 17, 2016 at 22:05

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