Start the Car With a Dead Key Fob Battery

Last updated: June 10, 2026

The Mercedes A-Class (W176, 2012–2018) uses a Mercedes SmartKey, and depending on how your car was specified it either has a conventional ignition slot or full KEYLESS-GO push-button start. Either way, a flat coin-cell battery in the fob does not strand you — the immobiliser reads the key through a passive transponder that needs no battery at all, and a metal emergency blade lives inside the fob to get you into the cabin. This guide walks through both, plus the coin-cell swap so it does not happen again.

Start the W176 with a dead key fob battery

The transponder chip inside the SmartKey is powered inductively by the car, so it keeps working when the coin cell is flat. The job is simply to get the key close enough to the reader coil.

1. Sit in the driver’s seat with the doors closed and your foot firmly on the brake pedal.
2. If your A-Class has the conventional ignition slot to the right of the steering column, push the SmartKey fully into it and turn as normal — the slot is itself the antenna and starts the car with a dead battery.
3. If your car has KEYLESS-GO with a push-button START/STOP switch, hold the SmartKey flat against the start button so the key body touches it, then press the button through the fob.
4. On KEYLESS-START cars the round START/STOP button pops out to reveal a slot — pull the button out and push the SmartKey into the exposed slot, then press to crank.
5. The dashboard will show the key symbol briefly, then start. Replace the fob battery at the first opportunity.

Unlock the door with the emergency key blade

If central locking will not respond, the A-Class hides a mechanical key inside the fob and a matching lock barrel in the driver’s door handle.

1. Slide the small release catch on the back of the SmartKey and pull the metal emergency blade straight out.
2. On the driver’s door handle, look for the keyhole. On the W176 it sits under a small cover cap at the rear edge of the handle — pop the cap off (some cars expose it by inserting the blade tip and levering the cap free).
3. Insert the blade and turn to unlock. Opening the door this way may set off the alarm; it stops once the SmartKey is recognised inside.
4. Get in, then use the start procedure above. Push the blade back into the fob until it clicks.

Why the immobiliser still recognises the key

The W176’s electronic ignition switch (EIS/EZS) energises a coil around the start button or ignition slot. When the SmartKey is held against it, that field powers the transponder and the key transmits its rolling security code with no help from the coin cell. This is exactly why holding the fob against the button works even when the remote-lock buttons have gone completely dead.

Replace the key fob battery

The W176 SmartKey (the slim “chrome” arrow-style fob) typically takes two CR2025 coin cells stacked in the fob; some earlier keys take a single cell. Always check what you remove.

1. Slide out the emergency blade — this releases the cover.
2. Use the slot left by the blade to prise the back cover off the fob.
3. Note the orientation of the cell(s), then lift them out.
4. Fit fresh CR2025 cell(s) the same way up (positive/printed face as removed), refit the cover and reinsert the blade.
5. Test from a few metres away — the lock buttons should respond instantly.

Frequently Asked Questions

Does my W176 A-Class have a slot or a push button? Both were sold. Pre-facelift cars commonly have the ignition slot; KEYLESS-GO cars have a push button. On KEYLESS-START cars the button itself pulls out to reveal a key slot.

How many batteries does the A-Class fob take? Most W176 SmartKeys use two CR2025 cells; a few earlier fobs use one. Open yours and match the count and size.

The alarm sounded when I used the metal key — did I do it wrong? No. Mechanical entry trips the alarm by design because the car cannot “see” the fob. It silences the moment the SmartKey is detected in the cabin.

Where is the keyhole on the driver’s door? Under a small cover at the rear of the driver’s door handle. Lever the cover off to reach the barrel.

My fob still does nothing after a new battery — what now? Confirm the cells are seated and the right way up. If the remote still fails the fob may need resynchronising or repair, but the car will still start by holding the key against the button or slot.

If a warning lamp stayed on after a flat key or weak 12V battery, you can look up what the code means on our sister site autodtcs.com.