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Important tip for owners regarding key FOB signal interference

This past Sunday I had a slightly distressing experience with my Model S. I drove up to a hiking trail near San Francisco and parked in a parking lot at the top of the mountain where there also happened to be several nearby cell towers. To my surprise, when I returned to my car I could not open the vehicle with the key. Pressing the buttons and moving near the car didn't work. Even putting the key on the windshield near the passenger side windshield wiper didn't work (which in retrospect makes sense since the issue was wireless interference and not a low key battery). Eventually I used the key to unlock the doors by holding it inches from the outside of the rear windshield glass. I didn't think about using the mobile app at the time but that would have probably worked too.

I then got in the car and tried to start it. No matter where I put the key in the interior of the vehicle the car would not recognize the key to allow me to start the car. I was on the phone with Tesla roadside assistance (who were very helpful) and they had me move the key to every inch of the interior including putting the key on top of my parcel shelf to no avail. The key would not be recognized and I could not start the car by putting my foot on the brake pedal. Thankfully we eventually discovered that if you put the key on the OUTSIDE of the rear windshield at the base of the glass, the car DID recognize the key and someone sitting in the driver's seat could press the brake and start the car. However I also discovered that if you leave the key on the rear windshield where it would be recognized and start the car by pressing the brake, the moment you get out of the driver's seat to retrieve your key before driving off, the car turn's off again! Thankfully I had a passenger with me that could retrieve the key while I stayed in the drivers seat after starting the car.

If the car had a passcode protected ignition, accessible on the touchscreen, that would have made the ordeal much less trouble and would also ensure that someone traveling alone would be able to start the car in the same situation I was in. If any wireless engineers in the Bay Area want to do some testing, I'd recommend going to the address below where the cell tower interference caused my issue.

900 Radio Rd‎
Brisbane, CA 94005

All in all, this problem I experienced would have affected any car's keyless entry and ignition system I'm sure, but Model S is unique (as far as cars I've driven) in that there is not really a convenient backup method for starting the car. My key has worked flawlessly since I left that specific location so I'm very sure that the issue was wireless interference and not a low battery in the key. I now remember from a previous trip to that location in my BMW that the keyless entry system also didn't work but my BMW had a physical key built into the FOB as a backup. Anyway, keep the rear windshield trick in mind in case you're ever stuck because of key signal issues

I'm curious what frequency the key is operating at.

What you encountered is probably what radio people call "desense". A nearby strong radio signal at one frequency will overwhelm a radio receiver at a different frequency and make it unable to pick up a signal.

Seems weird that a cell tower would have enough oomph to make that happen. You were probably parked in exactly the wrong spot. :-/

The standard FOB frequency is 315 MHz in North America. 315 MHz is way below the frequencies used in the cell tower (The 700 MHz LTE band is the lowest used right now).

As AlbertB points out, the very high levels near the tower likely overwhelms the receivers with a high-level sub-harmonics. Often cell towers also have multiple antennas for different frequencies, that can also cause interference points that are stronger at specific physical points, and the location you parked might have been at one of these junctures.

Your car turns off after putting it in Park. There may be a way to work around your particular situation.

Instead of putting the car in Park you can put it in Neutral and set the emergency brake. This technique leaves the car "On".

People have been able to drive off without their fobs by accident. (There was another thread on this.)

Lay a sheet of aluminum foil over the fob?

@ Captain_Zap. That's a good trick!

@Brian H

The tinfoil goes on your head, not over the fob;-)

Did you try putting the fob in the cup holder? That is where the sensor is if the battery is dead.

There have been similar reports from Mt. Wilson in Southern California, which also has a large concentration of transmitting towers. Good to know about the outside of the rear windshield trick.

@Captain_Zap: I didn't know you could do that! I assume you must set it through the setting/config screen then? Just pushing the stalk will only engage the EB as long as you are holding it in. I will look for it tomorrow.

Teslatap wrote a good analysis. De-sense is indicated due to the proximity of the tower. Sub harmonics and beating can move some energy into the fob's spectrum.

That said, I think we need a better plan b when rf interference exists.

An ohmic, direct contact dock in the cabin could help, but my favorite is a thru the glass capacitive pin pad in the corner of the windshield.

If you lose your fob, and your phone, you should have a way to get in and drive off based entirely on a knowledge test.

You can wire millions of dollars this way, so you should be able to start your car too.

This is cheap ($3) to do, and can be an add-on to the currently tooled design.

Hope TM puts some thought into a backup method. Since we don't want to regress to a mechanical key, there is a chance to do some meaningful innovation here too.

When we put the car in neutral and set the parking brake we use the touch screen.

We use this technique every day when we hop out of the car at the mail boxes. We also hit the button to raise the suspension at the same time so we have good clearance when we turn into our steep driveway.

Were they cellphone or microwave towers? Microwave signals are much stronger than the relatively weak cellphone signals. The cluster of microwave towers north of Calistoga (Mt. St. Helena?) are so powerful there is a very uneasy feeling to be experienced at the base of the towers.


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