These instructions apply to the Citroën C3 Mk3 (SX) 2016-2024.
For other models, please choose your vehicle here.
If your Citroen C3 Mk3 (2016–2024) has Keyless Entry & Start and the fob battery goes flat, the car stops detecting it — but there is a built-in backup so you can still unlock and start.
How to start a Citroen C3 with a dead key fob battery
- Slide out the hidden mechanical key blade from the fob and use it to unlock the driver’s door.
- Get in and hold the fob against the marked start-up zone (on the C3 this is typically by the steering column / start button area shown in the handbook).
- Manual: select neutral and press the clutch fully. Automatic: select P and press the brake.
- Press the START/STOP button to switch on and start the engine.
Replace the keyless fob battery (a CR1620 coin cell) as soon as you can so normal keyless operation returns.
Frequently asked questions
What if my C3 uses a key you turn in the ignition?
Then a flat fob battery won’t stop you starting — the transponder is read when you turn the key. Only the remote buttons stop working until you fit a new battery.
Where is the hidden key blade?
Inside the fob itself — slide the catch and pull the metal blade out. It unlocks the driver’s door when the central locking won’t respond.
If the immobiliser or key warning light stays on after a new battery, you can check the stored fault code on autodtcs.com.
Independent guide — not affiliated with Citroën. Always follow your official service manual and safety precautions. Read our full disclaimer.