Dead keyless remote seems to be detected

Usually that means the battery is dying inside the keyfob. Replace the battery to get rid of that message.

 

thanks.

 

did you notice the electronics in the key?

 

There actually are electronics in the key.

Let me start by describing what happens with the keyed Prius - the one where you actually have to insert the key into the lock on the steering column and turn it in order to start the car. In the old days, these kinds of keys simply closed an electrical switch to turn on the ignition system and run the starter motor. The problem with this was that it was too easy for someone to just "short" the wires to start and run the car without a key.

The next step was a mechanical interlock, so that the steering column couldn't be turned unless the key was actually in the lock. Unfortunately these systems were also pretty easy to defeat using a screwdriver and whatnot.

So for the last 10 or 15 years cars have had electronic "immobilizers" to ensure that the car won't run unless the key is present. The immobilizer cuts out critical parts of the system such as the ignition coil and fuel pump so that there's no simple way to bypass it. When you put the key into the lock it disables the immobilizer so that the car can run. This is done electronically - the key contains a transponder chip and there is a special reader in the steering lock. The transponder chip is passive - it works without needing a battery or power connection. The reader in the steering lock emits an EMF pulse that produces a small surge of current through the antenna in the key's transponder chip, and the transponder chip uses that to respond with a small radio signal. Because the key doesn't have it's own power source, it can only generate a very weak radio response, and so it needs to be very close to the reader (within an inch or two).

Early transponders used a fixed code and so it was fairly easy to get duplicate keys made. But of course that technology soon became available to thieves, so modern transponders use a series of rotating encrypted codes - more secure, but with the downside that the car needs to be programmed to accept new keys and this can usually only be done at the dealer.

Now in addition to the transponder, even the low end Prius models use a key that can remotely lock and unlock the doors. That lock/unlock function uses a standard radio transmitter that's powered by a battery. But that function is completely separate from the transponder chip that's required to disable the immobilizer. The transponder chip is still there, and it still doesn't need a battery to operate because it's powered by a pulse emitted by the reader in the car.

OK - so back to the keyless entry fob. These fobs have a battery-powered radio so that they can exchange an encrypted rotating code with the car to unlock the doors and start the car. This has to be battery powered because the whole point is that you can leave the fob in your pocket, which means that the radio needs more power than can be gotten by a passive transponder chip. But they also have the transponder chip in them, and if the battery fails then the car's immobilizer can still be disabled using that chip. And since the chip is passive and gets its power from the reader's EMF pulse you have to hold the fob right next to the reader in order for the car to be able to read it.

The Prius is actually very sensible by putting the transponder reader next to the "Start" button - it seems a little more intuitive that you'd hold the key fob near the "Start" button if the battery is weak or dead. By comparison some other cars have the transponder reader in non-obvious locations such as in a storage compartment. Prius owners with dead batteries in their keyless fobs are often able to stumble into the correct procedure to get the car started, while other some other car owners really don't have a chance unless they've read the manual to familiarize themselves with the obscure location that they have to put the fob in for the car to read it.