Key fob battery is dead and car won't start

Symptoms

• Fob doesn't unlock doors from a distance or at all
• Pressing the start button shows a 'key not detected' message
• Fob buttons may not light up or respond
• Spare fob (if you have one) may work fine

Likely causes

• CommonDead or very weak coin-cell battery in the fobThis is the single most common cause and the easiest to check.
• PossibleFob internal contacts corroded or dirtyMoisture or age can corrode the small battery contacts inside.
• PossibleBackup key slot not being used correctlyMost vehicles have a way to start with a dead fob if you know the right procedure.
• Less commonFob physically damaged (dropped, water exposure)Physical damage can affect the internal circuit board even with a fresh battery.
• Less commonAntenna/receiver issue on the vehicle sideLess common, but a vehicle-side fault can mimic a dead fob.

What to do

1. Check your owner's manual for the exact 'dead fob' backup starting procedure for your specific model.
2. Most vehicles have a hidden slot (often in the console or near the steering column) where you hold or insert the fob to start with a dead battery.
3. Replace the fob's coin-cell battery, which is usually accessible by prying open the fob's back cover or sliding out the mechanical key.
4. After replacing the battery, test both unlocking the doors and starting the car.
5. If you have a spare fob, try that first to confirm the issue is the fob and not the car.
6. Check the fob for any signs of water damage or physical wear if a new battery doesn't help.
7. If the backup starting procedure and a new battery both fail, the fob itself or the vehicle's receiver may need service.
8. Keep a spare fob battery in your glovebox once you know the correct type for future convenience.

Tools and parts

• Small flathead screwdriver
• Replacement coin-cell battery

When to call a pro

If a fresh battery and the manual's backup procedure don't get the car started, a dealer or locksmith can check the fob and vehicle receiver.

Safety