Start the Car With a Dead Key Fob Battery

Last updated: June 10, 2026

The Mercedes GLA (H247, 2020–Present) is the second-generation compact SUV, sharing its push-button KEYLESS-GO architecture with the W177 A-Class and C118 CLA. There is no ignition slot, so a dead SmartKey is handled at the start button itself — hold the fob against it and the immobiliser reads the key through a battery-free transponder. A hidden metal blade gets you into the cabin. Here is the full sequence.

Start the GLA with a dead key fob battery

The transponder in the SmartKey is powered by the car, not the coin cell, so it still authorises a start with a flat fob battery. The job is getting it against the button’s reader coil.

1. Get in, close the door, and press the brake pedal firmly.
2. Hold the SmartKey flat against the round START/STOP button so the fob body touches it.
3. With the key held there, push the START/STOP button through the fob.
4. The cluster flashes the key symbol, the reader detects the transponder, and the engine cranks.
5. Fit a fresh coin cell soon — this is a get-you-going method.

Unlock the door with the emergency key blade

If approaching the door no longer unlocks the GLA, use the mechanical key folded into the SmartKey.

1. Slide the catch on the fob and pull the metal emergency blade out.
2. The driver’s door keyhole on the H247 is hidden under a cap on the door handle. Insert the blade tip into the recess at the rear of the handle and lever the cap off to reveal the barrel.
3. Insert the blade and turn to unlock. The alarm may sound — this is normal for mechanical entry.
4. Get in and start with the button method above. The alarm stops once the SmartKey is detected. Clip the blade back in.

Why holding the fob to the button works

The GLA’s electronic ignition module energises an induction coil inside the START/STOP button. With the SmartKey held against it, the transponder is powered and transmits the encrypted rolling code that authorises the start. The remote-entry buttons depend on the coin cell, but the start handshake does not — which is why a completely dead fob can still start the car.

Replace the key fob battery

The H247 SmartKey takes a coin cell — commonly a single CR2025, though some Mercedes keys carry two. Confirm against what you remove.

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

Frequently Asked Questions

The GLA 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 H247 SmartKey use? Usually one CR2025, but some keys take two. Open yours and match it.

Does the brake need pressing? Yes, exactly as in a normal start. No brake, no crank.

The alarm went off when I used the blade — is that a fault? No. Mechanical entry is treated as a possible break-in until the SmartKey is sensed inside, then the alarm silences itself.

New battery, but the remote still does nothing. Re-seat the cell and check polarity. If the remote stays dead the fob may need resyncing, but the car still starts by holding the key to the button.

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