A Mercedes key stops working in most cases because the coin cell inside the key is flat or sits on a bent contact, and a new CR2025 or CR2032 cell fitted the right way up restores the buttons.
Mercedes key not working is the fault where a Mercedes key no longer locks, unlocks or starts the car. The chrome SmartKey sends the lock signal by radio and by infrared, and the electronic ignition switch reads the key in the slot. Keyless Go models read the key from a pocket and start with a button. A key that fails on the buttons and still starts the car has a fault on the remote side, which is the common case. Five checks in order clear most cases, and seven causes explain the rest. The fault appears across the range, including the A Class, the C Class, the E Class, the GLA, the GLC, the Vito and the Sprinter. A fob battery fitted and tested costs from £25 in Liverpool, and a new SmartKey costs £150 to £450.
A car that ignores the key, a start with a flat key, the battery type, a battery change, the key not detected message, the infrared test, the buttons, a car that unlocks without a start, a key that does not turn, both keys, the emergency key, the red light, a used key, the dealer and the time for a new key raise fifteen further questions.
The list below gives four key takeaways on a Mercedes key not working.
- A flat coin cell is the most common cause, and a battery change does not erase the key from the car.
- A key that still starts the car has a remote fault, and a key the car refuses has a chip, switch or steering lock fault.
- Both keys failing on the same day points to the car and not to the keys.
- An auto locksmith tests the key at the car before any new key is made, with no visit to a main dealer on supported models.
What fixes a Mercedes key not working?
Five checks fix a Mercedes key not working, and the first two clear a flat coin cell, the most common cause. The list below gives the five checks in order.
- Press a button and watch the small red light on the key, which stays dark with a flat coin cell.
- Fit a new coin cell of the type printed on the old cell, a CR2025 or a CR2032, with the plus side facing the same way.
- Check that the metal contacts inside the key press on the new cell.
- Start the car with the key, in the ignition slot or at the backup position of a Keyless Go model.
- Try the spare key, which shows whether the fault sits in the key or in the car.
The backup position on a Keyless Go model is the slot behind the start button on older cars and a marked spot in the cup holder or the centre console on later cars, and the handbook shows it. A Mercedes key not working after all five checks has a damaged circuit or a fault on the car side.
What does each Mercedes key symptom mean?
Each Mercedes key symptom points to a different part, and a key that still starts the car has a fault on the remote side only. The table below matches four symptoms to the part at fault.
| What the key does | Where the fault sits | First step |
|---|---|---|
| Buttons do nothing, the car still starts | The remote side of the key | A new coin cell, then a resync |
| Buttons work, the car does not start | The chip in the key, the ignition switch or the steering lock | The spare key, then a test at the car |
| Key goes into the slot and does not turn | The key, the ignition switch or the steering lock | A charged car battery, then the spare key |
| Both keys fail on the same day | The car, not the keys | The 12 volt battery and the fuses |
The buttons and the start function run on separate circuits inside the key. The buttons draw on the coin cell, and the ignition switch powers the key in the slot with no help from the cell. A Mercedes key with silent buttons that still starts the car is a small repair, not a new key.
Why does a Mercedes key stop working?
A Mercedes key stops working because the key has no power, the key is damaged or the car does not hear it. The list below gives seven causes in order of frequency.
- A flat coin cell leaves the buttons without power.
- A wrong cell, a cell fitted upside down or a bent contact leaves a new battery unconnected.
- A worn button contact or a cracked solder joint stops one button or all of them.
- Water or a drop damages the circuit board inside the key.
- A key out of sync with the car sends a code the car no longer expects.
- A weak 12 volt car battery, a blown fuse or a failed receiver leaves the car deaf to every key.
- A failed ignition switch or steering lock refuses a healthy key.
The first five causes sit in the key, and the spare key separates them from the last two. A Mercedes key not working beside a spare that works is a key fault.
Why does it fail after a battery change?
A Mercedes key fails after a battery change where the new cell is the wrong type, sits upside down or misses a bent contact.
The chrome SmartKey takes CR2025 cells and many later keys take a CR2032. The two cells differ in thickness, so a thin cell in a deep holder loses contact. The plus side faces the same way as on the old cell. The metal contacts flatten with age, and a contact lifted gently with a fingernail grips the new cell again. A battery change does not erase a Mercedes key, since the key stays paired to the car. A key that stays silent with a fresh cell seated the right way has a failed circuit, and the infrared test with a phone camera confirms it.
How does a Mercedes key resync?
A Mercedes key resyncs in most cases once the key goes into the ignition slot and the ignition is switched on.
The car reads the key in the slot and brings the remote code back in step, so the buttons work again after the key comes out. Some older models add a sequence with the lock and unlock buttons, and the handbook gives the sequence for each model. Keyless Go keys follow the handbook steps in the same way. A Mercedes key that stays out of step after a resync has a fault that a resync does not reach, such as a failed circuit in the key or a failed receiver in the car.
Who fixes a faulty Mercedes key?
An auto locksmith fixes a faulty Mercedes key where the key is at fault, and an auto electrician or a main dealer repairs a failed ignition switch, steering lock or receiver.
The work happens at the car. The coin cell, the infrared signal and the radio signal are tested, the spare key is tried and the fault codes are read through the diagnostic port. A key with a worn button or a cracked case is repaired, and a key with a failed circuit is replaced. ADC Locksmith Liverpool carries out Mercedes car key replacement in Liverpool from £120 with no call out fee and no VAT, 24 hours a day. Craig Evans holds SERMI approval, the accreditation for access to the security data of the car maker, and checks photo ID and proof of ownership first. Models on the FBS3 security system take a new key at the car. Some later FBS4 models take a key ordered through a main dealer, which Craig confirms from the registration before any charge.
What does a Mercedes key repair cost?
A Mercedes key repair costs from £25 for a fob battery fitted and tested, and from £45 for a button or case repair in Liverpool. The table below compares five outcomes of a Mercedes key not working by price.
| Outcome | Price | Who does it |
|---|---|---|
| New coin cell fitted at home | £2 to £5 | The driver |
| Fob battery fitted and tested at the car | From £25 | ADC Locksmith Liverpool |
| Button or case repair | From £45 | ADC Locksmith Liverpool |
| New SmartKey from a mobile specialist | £150 to £450 | ADC Locksmith Liverpool |
| New key from a main dealer | £500 to £900 | Main dealer |
Most cases end at the first two rows. ADC Locksmith Liverpool gives a fixed Mercedes key price from the registration on the call, before travel.
What else do Mercedes drivers ask about keys?
Mercedes drivers ask about keys in fifteen further questions on a Mercedes key not working. A car that ignores the key, a start with a flat key, the battery type, a battery change, the key not detected message, the infrared test, the buttons, a car that unlocks without a start, a key that does not turn, both keys, the emergency key, the red light, a used key, the dealer and the time for a new key are the fifteen topics, and the questions below answer each one.
Frequently asked questions
Why is a Mercedes not responding to the key?
A Mercedes is not responding to the key in most cases because the coin cell in the key is flat.
A weak car battery, a blown fuse and a failed receiver are the causes on the car side.
How does a Mercedes start with a flat key?
A Mercedes starts with a flat key pushed into the ignition slot or held at the backup position of a Keyless Go model.
The guide on how to start a car with a flat key battery covers other makes.
Which battery does a Mercedes key take?
A Mercedes key takes CR2025 coin cells in the chrome SmartKey and a CR2032 in many later keys.
The type is printed on the old cell.
Does a battery change erase a Mercedes key?
No, a battery change does not erase a Mercedes key, since the key stays paired to the car.
A key that fails with a fresh cell has a contact, circuit or sync fault.
Why does a Mercedes show key not detected?
A Mercedes shows key not detected where the Keyless Go key is too weak to answer the car.
The key not detected warning guide covers the message on every make.
How is the infrared on a Mercedes key tested?
The infrared on a Mercedes key is tested with a phone camera pointed at the tip of the key.
A light on the screen at each button press shows that the key is sending.
Why do the buttons on a Mercedes key stop working?
The buttons on a Mercedes key stop working where the coin cell is flat, a solder joint has cracked or a contact is worn.
The guide to a key fob not working covers every make.
Why does a Mercedes key unlock but not start?
A Mercedes key unlocks but does not start the car where the ignition switch or the steering lock refuses the key.
The key, the switch and the lock are tested before any part is changed.
Why does a Mercedes key not turn?
A Mercedes key does not turn where the ignition switch fails to read it or the steering lock stays engaged.
The fault sits in the key, the switch or the lock, so the key is tested first.
Why did both Mercedes keys stop working?
Both Mercedes keys stop working together where the fault sits in the car.
A weak 12 volt battery, a blown fuse or a failed receiver is the usual cause.
Why is the Mercedes emergency key not working?
The Mercedes emergency key is not working where the blade is worn or the door lock has seized from lack of use.
The blade opens the driver door only and does not start the car.
What does the red light on a Mercedes key mean?
The red light on a Mercedes key means the coin cell has charge and the button press was sent.
A key with no red light has a flat cell or a failed circuit.
Does a used Mercedes key work on another car?
No, a used Mercedes key does not work on another car in most cases.
A Mercedes key is locked to its first car and is rarely written a second time.
Is a Mercedes key programmed without the dealer?
Yes, a Mercedes key is programmed without the dealer on models with the FBS3 security system, at the car.
Some later FBS4 models take a key ordered through a main dealer.
How long does a new Mercedes key take?
A new Mercedes key takes one visit at the car on supported models.
A key ordered through a main dealer takes several working days.
Based in Liverpool and still stuck?
ADC Locksmith Liverpool repairs, programs and replaces car keys at the roadside across Liverpool and Merseyside, usually on the same day. Craig Evans holds SERMI approval and a 5.0 rating from 341 Google reviews. A call to 07731 544220 brings a fixed price before Craig travels.
Call 07731 544220