MB Medic Mercedes-Benz Repair Guides, Service, How To DIY
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In this guide (7 sections)
If you have a Mercedes-Benz with KEYLESS-GO, you may wonder how to start the car if the battery in the key dies. Pressing the Engine Start/Stop button does nothing if your key fob battery has died.
Problem
The Mercedes key battery has died, and you can’t start your car using the START ENGINE push button.
How to start a Mercedes-Benz with a dead key fob battery
If the battery in the KEYLESS-GO remote has died, follow these procedures to manually start your Mercedes-Benz. Check your Start/Stop button before you do anything. This procedure is for the cars whose button sits proud of the dashboard and pulls off to expose an ignition lock underneath. That covers a long run of the S-Class, C-Class, E-Class, ML-Class, GL-Class, GLE-Class, SL-Class and SLK-Class. It is not every car with a Start/Stop button. On the later cars the button is flush and fixed, it does not come off, and there is a separate backup slot elsewhere in the cabin. Pulling on a fixed button will damage the trim. If your button sits flush, skip to the next section.
Step 1
Remove the metal key from your Mercedes key fob to enter the car. Use it to unlock the driver’s door manually. The alarm will go off when you manually open the driver’s door. Don’t worry! The alarm should stop when you insert the key into the ignition, as described in the next few steps.
Next, remove the Start / Stop button from the ignition lock. You can quickly pull it from the ignition, and the button will pop out.
Step 2
Insert your smart key into the ignition. It is ok if the smart key battery is dead.
Step 3
Press the brake pedal and make sure the transmission is in the Park position.
Step 4
Turn the KEYLESS-GO key in the ignition to start the engine.  Release the key as soon as the engine cranks.
Make sure to save the Engine Start/Stop button to reinsert it once you change the battery in your Mercedes key fob. Â In the meantime, you must start your MB by inserting the key into the ignition.
Next time, remember to change the key battery as soon as you see the warning message on your instrument cluster saying “Key Battery Low.”
If your Start/Stop button is fixed and will not come off
On the later cars the button is flush with the dashboard and there is no ignition lock behind it. Do not try to pull it off. It is not designed to come out and you will break the surround. These cars still have a way to start with a dead key, but the backup is a slot somewhere else in the cabin, and where it sits depends on the model.
- On the C-Class, E-Class, S-Class and the AMG versions of them from around the 2019 model year, the fob goes into a backup slot rather than into the button.
- On the GLE and GLS from around 2019, and the G-Class from 2024, the slot is inside the front center console.
- On the GLC 350e the slot is in the cup holder area.
- On AMG roadsters and coupes from around 2017 the ignition lock sits at the back of the center console storage compartment.
Those locations are the common ones, not a complete list, so check the key section of your own owner manual before you go hunting. The giveaway is a fob shaped recess, usually with a key outline molded into it.
Once the fob is in the slot the sequence is the same as above: foot on the brake, transmission in Park, then press Start/Stop. If the engine still will not crank, press the brake pedal considerably harder than feels necessary and try again, because residual pressure in the brake system can leave the pedal switch short of the point it needs to reach.
Why does this work?
When inserted into the ignition, the key is powered by the inductive current. That borrowed power is enough for the key to transmit its rolling code to the EIS, the electronic ignition switch, which checks the code and releases the engine to start. Nothing about the process needs the key to have a working battery, because the car is supplying the power.
What to do next?
Change the battery on your Mercedes-Benz key fob as soon as possible. The battery is easy to change yourself. Instructions on how to change the battery on any Mercedes-Benz key fob can be found here:
After you replace the key battery, no programming is required. Once you insert the key in the ignition to start the car, it should function properly, including the door lock/unlock feature.
Does a dead key battery need a diagnostic scanner?
No. A dead cell in the fob is a two dollar problem and the car is built to start without it, which is the whole point of the backup slot. Nothing on this page calls for a scan tool, and it is worth saying that plainly rather than selling you something.
The question a scanner does answer is the one that comes after you fit a fresh battery and the key still will not start the car, or it starts only in the slot and never from your pocket. At that point the fault is no longer the battery, and there are three places it can live: the key itself, the EIS, or the antenna ring and KEYLESS-GO sensors that are supposed to detect the key from a distance.
The YOUCANIC UCAN-II Pro reads the EIS and will tell you whether the car logged a key not recognized fault. That single reading splits the problem cleanly. If the EIS recorded the key and refused it, the key or its coding is the issue. If the EIS never saw a key at all while you were standing at the door with it, the car was not listening, and you are looking at the antenna side rather than the key. Replacing fobs before you know which side it is on is how people spend several hundred dollars and still have the same car.
When the EIS itself is the fault
If the car will not recognize any key, including a known good spare, the electronic ignition switch is the usual suspect, and we have a full guide to EIS and EZS failures.
This is the point where the repair route matters more than usual. A replacement EIS has to be coded to the car and every key has to be matched to it again, so you leave with a programming appointment and often a bill for new keys on top of the unit. A board level repair of the original EIS keeps the coding and keeps your existing keys working, so there is no rematching to do. Module Medics repair the original unit, including FBS3 and key work, and send it back.
Related reading
Before you buy parts, scan every control module, not just the engine. It shows the root fault behind the warning lights and tells you whether the problem is still active. See how to do a full health check.
A member of my family has a MB C220CDi Classic SE which has developed a erratic starting problem, causing it to not start. The engine does not turn over yet all the panel lights are illuminated. It has a 5 month old battery purchased new. The ECU has been checked and found to be fault free. Apparently no fault codes are showing
and having been recovered twice from this non start mode. It does then fire up. The start relay has been changed as a precaution, having previously been starting, when on fitting the new start relay, it refused to start one more. Thinking it was possibly a faulty replacement, I refitted the original, and once again no start.
It has been recovered twice from a non start mode and then taken to two separate specialists who still have not located the problem. Has anyone got a possible solution to this kind of irritating problem.
I have a 2005 E 320 CDI With similar no crank issue. It is erratic works most of the time but seems worse in hot weather conditions. Removing the lid of front SAM fuse box and tapping starter and computer control relays whilst turning key has worked every time to start engine. Probably a ECU Issue will probably get worse over time. I’m hoping with cooler weather approaching problem will resolve until next summer. I have replaced both backup and starter batteries, new Bosch alternator voltage regulator and the two above mentioned relays with new MB dealer parts but problem still happens. Hot/cold intermittent electrical issues are common on older computer circuits. I can live with intermittent no crank until I can afford to replace ECU.