What Actually Happens If You Leave Your Key Fob Behind While Driving

Starting your vehicle these days involves considerably more technology than in the days of simply inserting and turning a mechanical ignition key. The physical key has given way to push-button start, with the presence of a wireless fob that acts as a digital key (often referred to as a smart key). The key fob serves multiple vehicle functions, including the comfort of remote start, keyless entry, remote door locking and unlocking, and cargo compartment release. Car manufacturers have had a lot of fun with key fob design, as well. Everything is designed to work seamlessly as we approach our vehicle, get in the driver’s seat, and push the start button.

The vehicle responds to either button input on the key fob or the physical proximity of the key fob to the car. By design, the vehicle cannot be started without the key fob. However, since the key fob is not connected to your vehicle like a physical key, it’s easy to see how it could be left behind after your car is started. You might drop off a family member who happens to have the key fob in their possession, or exit the running vehicle yourself and leave the fob sitting somewhere before you return. Thankfully, you can still drive your car this way under certain conditions.

Let’s take a look at how wireless key fobs operate. A number of monitoring processes are happening within a vehicle’s electronic control units before, during, and after the car is started. A specific key fob identification code is matched to the vehicle control units for secure operation. These controllers continuously monitor the key fob via radio frequencies to determine its proximity to the vehicle. These signals determine what functions are available and when, such as when to unlock the doors, or when to allow the vehicle to start. Sometimes you can even start the car with a dead key fob battery.

When the on-board controllers sense the key fob has left the vehicle, the driver is alerted that the fob is no longer detected. The type of indicator can vary by vehicle manufacturer. There may be an illuminated symbol displayed on the instrument panel, or a message such as “no key detected” displayed in the information center. The indicator or message can also be accompanied by a warning chime.

If the vehicle has already been started and the key fob has been left behind, the car will continue to run and drive without it. However, not having a physical key connected to the car presents a safety problem, since the engine can stay running without the fob present. For example, this has resulted in carbon monoxide deaths from vehicles left running in garages. There has been a push for the National Highway Traffic Safety Administration to mandate an auto-shutoff feature for when the car is parked a certain length of time, and some manufacturers have voluntarily added it.

Leaving your key fob behind while the vehicle is running also presents the possibility of locking yourself out. Automakers can usually unlock your doors remotely as an emergency service. This also can come in handy if your key fob stops working due to a dead battery or other malfunction. And mobile apps for newer cars can let you unlock the doors with your smartphone. In fact, some of the latest vehicles support digital keys stored in your Apple or Android digital wallet, letting you both unlock and start your car.

Review your owner’s manual to better understand how your car will respond without the key fob present, or when the key fob isn’t working. This includes understanding its warning indicators and how the auto-shutoff works, if the car has it. 

One more thing. While you can continue to drive without the key fob, unless you’ve activated that smartphone app the vehicle can’t be restarted without it if you press the start/stop button. To avoid being stranded, retrieve your key fob as soon as you realize it’s missing from your vehicle.