Do they use RFID or some other technology that permits me to leave the fob in my pocket and open the door and start the car without touching the fob? Does the fob send out a unique signal that is read by the car or is the car sending out a unique signal that is read by the fob? I would assume the latter, but I don’t know.
Great technology.
I want to say it’s RFID to unlock the doors, but IIRC there’s a bluetooth connection involved for other things.
I would thing, at the very least, it would have to be RFID for starting the car otherwise you wouldn’t be able to do that if the battery (on the fob) died. RFIDs are passive and don’t need their own power source.
Not all RFID is passive.
ETA: In fact, on my last few cars (2 MINIs and a Kia Optima), if the FOB battery dies, you have to use a key embedded in it to open the door and have to plug the FOB into the dash to start the car.
For any reasonable level of security, you have to have “rolling codes”. Each time the fob is used, it has to send a new code to the car. The sequence is pre-arranged per pair and appears random (hopefully*) to someone monitoring the signal.
You can’t do this with an passive RFID chip. The memory in the chip needs to be updated.
In addition, when you lose a fob, you can get a new one (even at places like WalMart) and it has to be programed for you own vehicle. Can’t do that with passive RFID either.
Different cars do it in different ways. My Tesla has an active remote part that allows me to enter the car and drive off without taking the fob out of my pocket. In case the battery in the fob dies, it also has a passive RFID. I then have to place the fob in a specific spot on the outside of the car to unlock it, and then place it in the cup holder to drive it. Those 2 spots are where the RFID sensors are, and they are very short range. I figure Tesla made them short range intentionally, due to the lack of rolling codes.
Mine (a prius) also works by requiring you to touch the FOB to the start button if the battery is low. They have several antennas and seem to be able to accurately (I only did very quick testing of this) determine if key is outside or inside car.
One of my nightmares is that I have a super powerful new battery keyfob - that is in my jacket - that for some reason I took off and left in garage. Key is only a couple feet from car and then I start the card successfully - get to Starbucks or wherever - and then realize I don’t have my key.
Apparently they have already thought of this - and on occasions where I have left car running and lept out to grab something (with passenger in car) - it does stay running - but does something (I think beeps and comes up with warning message).
But if I give key (while car is off) to friend standing right outside (literally hand it through window) - and attempt to start car - it doesn’t work. I didn’t try very many variations on this to see how close you needed to be.
Keep in mind that if you have a Bluetooth phone connected and do this - the other person in the car now has control of your phone 
It’s neat that it can have that awareness of where the key is located.
Over the winter, I started locking the door on my Prius when I exited by using the lock button on the door itself rather than the lock button on the fob so I wouldn’t use up the battery in the fob. However, this spring one day I took off my coat in the car after getting in. I had the fob in my coat pocket. Sure enough, when I arrived at my destination, I jumped out and locked the door with the lock button and left my coat inside. The car could sense I’d “locked” my key inside the car and unlocked the car for me. Very nice!
Yeah, but no one has really answered my question. What sort of radio technology is used to accomplish these neat feats?
If the fob is on my person, I don’t have push any buttons on the fob or the car. I just pull on the door handle and all of the doors unlock and I get in. I put my foot on the brake and push the start button, and the car starts. It’s obvious that the car and the fob talk to each other and pass along some sort of encrypted signal. Which one initiates the conversation?
Is the fob constantly sending out some signal, and when the car in proximity it reacts? I don’t think that is it.
I think that the car initiates the conversation when I tug on the door handle. If the fob is in proximity, it responds and the doors open. Likewise, when I push the start button, the car again initiates the conversation and the fob responds to allow the car to start.
So my assumption is that the battery in the fob is there because the fob is always in listening mode, ready to respond.
Anyone know for sure?
Omar Little
I don’t know the answer but your explanation makes a lot of sense. If the FOB or car was constantly pinging the FOB would soon run down it’s battery.
It makes more sense that a physical action on the car causes a transmission which the FOB responds to after listening. Listening is a lot more energy efficient than broadcasting so the only time you would want the FOB to broadcast is when it is replying to the car.
on edit - My FOB also has buttons so it can initiate commands to the car. But again, the button method is to use the broadcast function only when desired.
Then the second fob which lives in the house permanently would never work once the ‘code’ had changed on the primary.
My fobs (Mazda 6) came with a sort of dog tag with a number stamped on it. I apparently need to keep them safe incase I loose a fob.
I suspect there is a encryption key associated with it that the car understands.
I don’t know for sure. I do know that I don’t have to do anything to the car to make it unlock. As soon as I’m close enough the door handles present. My guess is the car always sends a low strength signal that only reaches a few meters beyond the car. The fob always listens for that signal (this takes little power). Once the fob detects the correct signal, i.e. the car it is matched to, it sends an encrypted message that the car receives and unlocks.
Actually, the car is constantly emitting LF radio signal thus “initiates the conversation” even before you pull the door handle. When the fob picks up the signal, it emits a response ID code signal and if it matches the vehicle’s transponder ID code, it sends a separate signal to the ECU to allow the car to be locked/unlocked, start its engine, etc.
The battery in the fob is used to listen for and to emit its own radio wave.
There is a call/response pairing between the car and the fob. The car’s system can initiate a new code which the fob receives, does an encryption thing with it based on it’s internal ID, and sends that.
The point is that the code/response is never the same twice. But it is quasi-predictable in too many cases.