These instructions apply to the Mercedes CLA Mk2 (C118) 2019-Present. For other models, please choose your vehicle here.
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The Mercedes CLA (C118, 2019–Present) — the four-door coupe and Shooting Brake built on the same platform as the W177 A-Class — is a push-button, KEYLESS-GO-era car with no ignition slot. When the SmartKey coin cell dies, you start it by holding the fob against the start button, and a hidden metal blade unlocks the cabin. This guide covers both, plus the battery swap.
Start the CLA with a dead key fob battery
The SmartKey transponder is powered by the car rather than its own coin cell, so it still authorises a start when the fob battery is flat. You just need it against the button’s reader coil.
- Get in, close the door, and press the brake pedal firmly.
- Hold the SmartKey flat against the round START/STOP button so the fob body touches it.
- With the key held there, push the START/STOP button through the fob.
- The cluster flashes the key symbol, the reader detects the transponder, and the engine cranks.
- Replace the coin cell at the first opportunity.
Unlock the door with the emergency key blade
The CLA’s frameless coupe doors still hide a conventional lock barrel; the blade lives in the SmartKey.
- Slide the catch on the fob and pull the metal emergency blade out.
- The driver’s door keyhole is concealed under a cap on the door handle — on the coupe-styled CLA it is deliberately discreet. Insert the blade tip into the recess at the rear of the handle and lever the cap off to reveal the barrel.
- Insert the blade and turn to unlock. The alarm may sound; this is normal.
- Get in and start with the button method above — the alarm stops once the SmartKey is detected. Refit the blade.
Why holding the fob to the button works
The CLA shares the W177’s electronic ignition architecture: an induction coil in the START/STOP button powers the SmartKey transponder when the fob is held against it, and the key then sends its encrypted rolling code to authorise the start. Remote locking needs the coin cell; the start handshake does not.
Replace the key fob battery
The C118 SmartKey takes a coin cell — commonly a single CR2025, though some keys carry two. Check what you remove.
- Slide out the emergency blade to release the rear cover.
- Use the gap the blade left to ease the back cover off.
- Note how the cell sits, then remove it.
- Press in a fresh CR2025, matching polarity and count, then refit the cover and blade.
- Test from a few metres — the lock buttons should respond at once.
Frequently Asked Questions
The CLA has no key slot — how does a dead fob start it? By holding the fob against the START/STOP button, whose reader coil powers and reads the transponder without any fob battery.
How many coin cells does the C118 SmartKey use? Usually one CR2025, but some keys take two. Open yours and match it.
Where is the keyhole on the CLA’s frameless door? Under a cap on the door handle, hidden for styling. Pop the cap with the blade tip and the barrel is beneath.
The alarm sounded when I unlocked manually — is that a fault? No. The car treats mechanical entry as a possible break-in until it senses the SmartKey, then silences itself.
New battery, but the remote still does nothing. Re-seat the cell and check polarity. If it stays dead the fob may need resyncing, but the car still starts by holding the key to the button.
If a warning lamp stayed on after the flat key, you can decode it on our sister site autodtcs.com.
Disclaimer: The information on this page is provided for general guidance only. Always follow your official service manual and safety precautions when working on your vehicle. We are not responsible for errors, omissions, or any damage resulting from the use of this information.
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