MB Medic Mercedes-Benz Repair Guides, Service, How To DIY
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There may come a day when the Start/Stop button on your Mercedes-Benz no longer starts the engine. This article covers the causes that stop a push button start from working, from a flat key fob to the electronic ignition switch, plus the emergency override that gets you moving in the meantime.
Two completely different things on a Mercedes are called Start/Stop, and owners land here looking for both.
The engine START/STOP BUTTON is the push button on the dash that replaces a turning key on cars with KEYLESS-GO. If yours does nothing, or the dash lights up and the engine never cranks, this page is for you.
ECO Start/Stop is the separate system that shuts the engine off at a red light and restarts it when you lift off the brake. When that stops working it is usually not a fault at all. The car disables it when battery charge is low, while the cabin is still reaching the temperature you set, before the engine is warm, or when the steering is turned. If that is what you are chasing, check the battery and its state of charge first, because a tired battery is by far the most common reason ECO Start/Stop quietly stops engaging.
In case of an emergency
To get your Mercedes started manually, grab the Start/Stop button and pull it out. Make sure the transmission gear is in Park  (you should see the P gear on the dashboard) and you are pressing the brake pedal. Insert the key into the ignition and start the car.
Possible Causes
Smart Key Fob Battery
A weak battery in the key fob is one of the most common problems that prevent the Start/Stop button from working. As the battery in the key start gets old, it will unlock/lock the doors, but it will not have enough power to send a signal to the car for the Engine Start function.
If your Mercedes push start does not work, change the key fob battery first. It is the cheapest thing on the list and the most likely. Follow this guide to learn how to change the Mercedes key fob battery.Â
Note that sometimes new batteries don’t solve the problem. This can be because the “new” batteries may have sat on the shelf of your local store for years and are also discharged. Check the production date on the battery pack before you install it.
The brake pedal is not pressed
Your Mercedes-Benz will not start when you press the START button without pressing the brake pedal. Only the dashboard lights will turn on. Make sure you are pressing the brake pedal firmly.
The vehicle is locked
If you are inside the car and have pressed the lock button on the key fob, you won’t be able to start the car. Press the unlock button on the key fob. This will unlock the car and allow you to start the engine with the Start/Stop button.
Interference
The owner’s manual lists radio interference as a cause, and it does happen: parking under high voltage power lines or beside a broadcast transmitter can swamp the signal from the fob.
Move the car and try again if you suspect that. In practice, though, the interference is far more often coming from your own pocket. A fob pressed against a phone, a second key fob, a wireless charging pad, or a thick bundle of keys can be enough to stop the car reading it. Take the fob out on its own, away from anything electronic, and try the button again. If it works that way, nothing in the car is broken.
Further diagnostics, in the order that actually narrows it down
The emergency procedure at the top of this page is also the single most useful diagnostic step on the car, and almost nobody uses it that way. Pull the Start/Stop button out, insert the key into the slot behind it, and turn it.
If the car starts normally with the key turned, everything on the wireless side has just been cleared. The starter, the ignition circuit, the brake switch and the immobiliser are all working, and your fault is either in the key fob or in the car’s ability to read it at a distance. That means a fob battery, a damaged fob, or the KEYLESS-GO aerials.
If the car still will not start with the key physically inserted and turned, the fob is not your problem and you can stop buying batteries. The fault is in the ignition switch, the starting circuit, or a precondition the car is refusing on.
If the dashboard lights up but the engine never cranks
The car has accepted the key and is refusing on a condition. Check these in order:
- Gear selector genuinely in P, not resting in N. Look at the gear display in the cluster, not at the lever.
- Brake pedal. Have someone stand behind the car while you press the pedal. If the brake lights do not come on, the brake light switch is your fault, and that same switch also blocks starting and blocks shifting out of Park. It is a cheap part and a well known failure on these cars.
- The fuse for the ignition start circuit.
- Battery voltage. A battery that still lights the dashboard can be far too weak for the car to arm the starter, and a Mercedes will often refuse outright rather than crank slowly, which makes a flat battery look like an electronic fault.
If the cluster shows Key not recognized
Try your second key before anything else, because that one swap tells you whether the problem is the key or the car. If the spare works, the first key is at fault. If neither key is recognized, the car has stopped reading keys at all, and the electronic ignition switch is the usual reason.
If nothing happens at all
No cluster, no chime, no response of any kind. That is power or the ignition switch, not the fob. Start at the battery terminals and the ground straps.
A scan tool cannot press your brake pedal, but it can show you two things that otherwise cost an afternoon. It reads whether the electronic ignition switch has logged a key not recognized or key authorization fault, which separates a fob problem from a switch problem. And it displays the brake pedal switch signal as live data, so you can watch the state change while someone presses the pedal. That one reading confirms or clears the brake light switch in seconds.
The YOUCANIC UCAN-II Pro reads these Mercedes specific control units rather than only generic engine codes, which is exactly what a basic code reader misses here, and it covers nearly everything Xentry does apart from online VIN coding.
If neither key is recognized, or the car is dead one morning and perfectly normal the next, the electronic ignition switch, called the EIS or EZS, is the usual culprit on these models. It is also where owners spend the most money unnecessarily, because a dealer replacement means a new switch plus key and control unit matching to the car. Module Medics repair the original EIS at board level and return your own unit, so your existing keys keep working and nothing has to be matched again. They also handle key copying and FBS3 work when a key itself has failed.
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.
