Start the Car With a Dead Key Fob Battery

Last updated: June 10, 2026

The Mercedes A-Class (W177, 2018–Present) is a fully push-button, KEYLESS-GO-era car — there is no ignition slot to drop a key into. That worries owners the first time the SmartKey battery dies, but the W177 is built to handle it: hold the fob against the start button and the immobiliser reads it through a battery-free transponder, and a hidden metal blade lets you into the cabin. Here is the whole sequence.

Start the W177 with a dead key fob battery

The chip inside the SmartKey runs on energy the car beams to it, so it still talks to the immobiliser even with a flat coin cell. You just need the key close to the start button’s reader coil.

1. Get in, close the door, and press the brake pedal firmly — the A-Class will not start without the brake down.
2. Take the SmartKey and hold it flat against the round START/STOP button so the body of the fob physically touches it.
3. Keeping the fob pressed there, push the START/STOP button through the key.
4. The cluster flashes the key symbol, the reader picks up the transponder, and the engine cranks.
5. Drive as normal, then fit a fresh fob battery soon — this is a get-you-home trick, not a long-term fix.

Unlock the door with the emergency key blade

If touching the door handle no longer unlocks the car, fall back to the mechanical key folded inside the SmartKey.

1. Slide the release catch on the fob and pull the metal emergency blade out.
2. The driver’s door keyhole on the W177 is concealed under a cap on the door handle. Insert the blade tip into the small recess at the rear of the handle and lever the cap off to reveal the lock barrel.
3. Insert the blade and turn to unlock the door. The alarm may sound — this is normal for mechanical entry.
4. Get in and start the car using the button method above. The alarm stops once the SmartKey is detected. Clip the blade back into the fob.

Why holding the fob to the button works

The W177’s electronic ignition module energises an induction coil built into the START/STOP button. Bringing the SmartKey into that field powers its transponder, which then sends the encrypted rolling code that authorises starting. The remote-lock function needs the coin cell, but the start handshake does not — that is the whole reason this emergency method exists.

Replace the key fob battery

The W177 SmartKey takes a coin cell (commonly a single CR2025, though some keys carry two — check yours). Replacing it is a two-minute job.

1. Slide out the emergency blade; this unlocks the rear cover.
2. Use the gap left by the blade to ease the back cover off the fob.
3. Note how the cell sits, then remove it.
4. Press in a fresh CR2025, matching the polarity and count you removed, then refit the cover and the blade.
5. Step back a few metres and test the lock buttons — they should respond at once.

Frequently Asked Questions

There is no ignition slot — how can a dead fob start my W177? By holding the fob against the START/STOP button. The button contains a reader coil that powers and reads the key’s transponder without any fob battery.

How many coin cells does the W177 SmartKey use? Many use one CR2025; some carry two stacked cells. Open the fob and match exactly what comes out.

Do I need the brake pedal pressed? Yes. As with normal starting, the brake must be down for the engine to crank.

The alarm went off when I unlocked with the blade — is that a fault? No. The car treats mechanical entry as a possible break-in until it senses the SmartKey inside. It silences on its own.

My fob still will not lock the doors after a new battery. Re-seat the cell and confirm polarity. If the remote stays dead the fob may need resyncing or repair, but the car will still start by holding the key to the button.

If a dashboard warning light stayed on after the flat key, you can decode the fault on our sister site autodtcs.com.