How to Start a Mercedes-Benz with a Dead Key Fob
Starting Your Mercedes-Benz
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To start a Mercedes-Benz that uses the tapered SmartKey style, the engine Start / Stop ignition button must be removed. It appears to ‘float’ above the dashboard and is easily pulled away from the ignition lock. Insert the tapered end of the KEYLESS-GO key in the ignition lock and turn it to start and operate old school style.
Beginning in 2017, the ignition lock in AMG roadsters and coupes is found at the back of the center console storage area, as shown in the image. Insert the tapered key fob into the ignition lock and turn it to start and operate old school style.
In 2019, the AMG gets the new SmartKey, described next.
AMG Back-up Slot
Models using the new, rounded SmartKey fob styles , shown to the left, require that the fob be placed in a back up location or slot. These models, as of the 2019 model year, are the AMG, C-Class, E-Class and S-Class.
Warning! Do NOT attempt to remove the Start / Stop ignition button!
C-Class Back-up Slot
E-Class Back-up Slot
The back up slot in each model is found at the very front of the center console. Each slot is slightly different depending on the model, and an image from each model is shown here.
Each slot is marked by the image of a metal key with lines radiating from it, except for the C-Class, which has an image of the SmartKey fob itself (left).
S-Class Back-up Slot
Make sure that the slot is empty. Remove the SmartKey fob from the key ring and slide or place the fob into the slot with its buttons facing up. You can now start the car as normal. The car will recognize the key fob even if the fob’s internal battery is dead.
The new SmartKey fob style had spread through nearly the entire product line by the 2019 model year. However, the mark (the image of a metal key with lines radiating from it) is found in or near cup holders, which are at the front of the center console. See the two images at left. The 2021 S-Class Maybach uses the rear cup holder as well (second image) as well as a newer key fob. It is similar to the one shown above.
Also in 2019, Mercedes-Benz introduced the first of its electric and electrified models, the GLC 350e. And while the mark is also found in the cup holder, it too uses the image of the SmartKey (see the image to the right), like the updated C-Class models noted above. This will likely take over all model lines with time.
Finally come a small number of exceptions. Since 2019, the GLE and GLS SUVS use a slot, with the mark of a key with lines radiating from it, is found inside the center console storage area in the front. G-Class SUVs joined them in 2024. See the image to the left. Drop the key fob into the slot and start the vehicle as normal.
Note: For reasons unknown, all the Mercedes models we’ve tested have s light delay built in after the start/stop button is pushed and the key fob being recognized. Not to worry…
You can see videos of the open and start processes here, as well as key fob battery replacement.
Note: If you have made an attempt to start the vehicle without success, there may be residual pressure in the brake system and a new warning light may appear (left) or text message to Step on the Brake. Press the brake pedal very, very hard when trying to start the vehicle again to be sure that the release switch behind the pedal is engaged. Or wait a few minutes and try again.
… Return to Part I, Getting in
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Remember that only proper service and repair procedures will ensure the safe and reliable operation of your car. In addition, proper safety procedures and precautions, such as the use of safety goggles, the right tools and the equipment should be followed at all times to eliminate the possibility of personal injury or improper service which could damage the vehicle or compromise its safety.
These posts are for information sharing purposes only, and should not be used in lieu of an OEM service manual or factory authorized service procedure. We are not in the auto repair business nor do we publish automotive service manuals. Nothing we include on these pages and posts has been reviewed, approved or authorized by any vehicle manufacturer.
Technology is always changing and what is current and accurate today may be literally out-of-date and inaccurate tomorrow. And when it comes to the current state of flux in the auto industry, nothing is more true.
The author has 25 years of automotive experience and has assembled the most extensive collection of symbols and warning lights anywhere (over 1,000!) and can help you open and start any keyless start car with a dead key fob battery. BA, St. Joseph’s College, ME: MS, Rensselaer Polytechnic Institute, NY
Early Audi models use a slot in the dashboard but do not use a Start/Stop button. The fob itself is pushed to start or stop the car. If the key fob battery is dead, the vehicle will already recognize the fob when inserted.
In models that do not feature a slot for the fob, look for the symbol of a key with lines radiating from it, as shown. It will be to the right of the steering column on the dashboard (Audi) or on the right side of the steering column itself (VW).
In 2018, the mark is moved to the driver’s side cup holder in the Audi A5 and Q5. This is also true of the A3 and Q3 beginning about 2021. The all electric eTron GT uses this mark as well. The mark is smaller and shaded and so a bit difficult to see. As with the Q7, the key fob can be dropped into the cup holder, and starting the car becomes a one hand (and one foot) operation. Press the Start/Stop button while stepping on the brake. It took two tries to before the key fob was recognized in our test car, but it did work.
However, in 2019, the A5 now has the mark at the front of the center console storage area, as shown to the right. Again the key fob can be dropped onto the spot, and starting the car needs only a single hand (and foot). Press the Start/Stop button while stepping on the brake. Our test car started first time in this case.
The new (2019) Q8, and now the Q7, also has the marking in a cup holder. It is the front one in this case. However, we noted a slot between the cup holders adjacent to the 12 volt power outlet and shaped very much like the key fob, Sure enough, the fob was recognized when placed inside the slot.
Models are now being electrified. The e-tron GT uses the front cup as in the images above. However, the original e-tron and e-tron Sportback the mark is found in front of the of the cup holders (right). We have not had access to all electric models, but a back up location will likely be found in or near the cup holders.
By 2023, the A4 and S4 models sport the mark on the center console, just in front of the console storage area.
The mark in Volkswagen’s new ID4 EV, the mark is moved to the front area of the center console. This follows the trend in Audi models. Originally, the front area is for storage and has removeable spacers. We found the mark by removing the front spacer (first image). Later, the same area was converted to cup holders. The mark is found in the first cup holder (closest to the driver) (second image).
Beginning with the 2020 XC-90 redesign and the newer key fob style, plus Polestar EV models, a sensor location is provided for the key fob. It is found in the second cup holder in the center or tunnel console. An illustration of a key with lines radiating from it can be seen at the bottom of the cup holder.
Note that you will find the mark inside the small center console of the Polestar 2 (right), which is also the location of the second cup holder! Place the key fob in the cup holder. The vehicle will recognize the fob even if the fob’s internal battery is dead. The alarm will be turned off and the car can be started as normal, by turning the Start / Stop switch to the Start position while depressing the brake. Note that the Polestar 2 is started, or powered up, by stepping on the brake.
In the Volvo XC-40, the company’s first all electric model, and subsequently the C40 and EX40 EVs, the mark will be found inside the center console on the bottom and to the front (right). It too features the mark of a key with lines radiating from it. Place the key fob on the mark. The vehicle will recognize the fob even if the fob’s internal battery is dead. The alarm will be turned off and the car can be started as normal.
So, in both the Polestar 3 and 4, the key card is placed on the spot used to charge your smart phone (right), which is at the front of the center console. The key fob for the Polestar 3 can be recharged on this spot, the card reader, as well.
In older, second generation keyless start Volvo models, an ignition slot comes into play. The slot will be found either alongside or below the Start/Stop ignition button. Insert the key fob into the ignition slot. This will turn off the alarm and allow the vehicle to be started as normal by pressing the Start / Stop button while depressing the brake.
The vehicle will recognize the key fob even if the fob’s internal battery is dead.
In first generation keyless start models, the ignition switch is turned in normal conditions, similar to key start vehicles. There is a dummy key that needs to be removed. Press the the button found on the side of the dummy key, which frees it to be pulled away. The key fob can then be inserted in its place and the vehicle can be started (right).
If there is a dummy key, it will need to be removed in order to use the key fob in the ignition. The owner’s manual holds a somewhat complicated set of removal instructions that will leave you exposed and vulnerable for far too long. Ignore the instructions and do the following.
Make two attempts to start the car normally. The car’s instrument panel will then note that the dummy key can now be pulled away. Once this is done, the vehicle’s key fob can be inserted into the ignition switch and the car started normally. The fob will be recognized even if the fob’s internal battery is dead.
Lay the key fob, buttons up, in the compartment in the front of the center console to the left of the cup holders. Turn the ignition dummy key to the start position (position 2) and back to off. Turn the dummy key back to the start position a second time and the vehicle will start. The instrument panel will also acknowledge that the key fob has been recognized.
In Chrysler and Dodge vehicles built prior to 2009 and using the rectangular-tipped key, the Engine Start / Stop button must be removed from the ignition switch for key fob use. With the ignition in the Lock position, insert the metal part of the emergency key (or a finger nail!) under the chrome button (above right) and gently pry the button loose. It comes away very easily. Note that the button itself appears to float 
just above the dash.
In 2023, Dodge introduced the Hornet model, and it uses a very different back-up start method. The key fob needs to be placed in the front cup holder (rightt). The fob may need to be removed from a key chain to make this work.
There are several methods for starting a Honda or Acura vehicle. To start a Honda and the newest Acura models lacking a slot, touch the “H” in the Honda logo or the “A” in the Acura logo on the remote control to the Start / Stop button until the small indicator light in the button begins to flash. For the next 30 seconds, the vehicle can be started using the Start / Stop button normally. You can also simply press the Start / Stop button with the remote control itself, while stepping on the brake of course.
exactly the same.
In earlier Acura models (newest models are above), there is a slot for the remote control (key fob) to the left and below the Start / Stop button. Insert the remote and use the Start / Stop button to start the vehicle as normal.
In the oldest keyless start Acura models, there is a cover on the ignition switch that must be removed. This will allow the use of the mechanical key built into the key fob. To remove the ignition switch cover, slip the built-in key into the parallel slot underneath the cover. Push the key in while pull it towards you at the same time and the cover will come off. Once the cover is removed, the mechanical key can be inserted and turned to start the vehicle.
Two methods exist in Nissan and Infinity vehicles. Some models use method 1. In this case, insert the Intelligent Key in the port provided. It will be found to the left of the steering column. It might be horizontal (first image) or vertical (second image).
port or with the buttons facing the door in the vertical port. Make sure the key ring side is out.
If a port does not exist, method 2 comes into play. Simply touch the ignition switch with the Intelligent Key (a chime will sound). Push the ignition Start / Stop button while depressing the brake pedal within 10 seconds after the chime sounds to start the engine.