Why has my car key fob stopped working?

In most cases the battery. A fob battery typically lasts two to four years, and the failure is usually gradual — reduced range first, then intermittent operation, then nothing.

The batteries are almost always inexpensive coin cells, commonly CR2032 or CR2025, and most fobs open with a small screwdriver or a coin along a seam. The correct type is printed on the cell you remove.

If a new battery does not fix it, work through the alternatives.

Test the second fob. If both have failed simultaneously, the problem is more likely in the car than the fobs.

Radio interference is a genuine and underrated cause. Fobs operate on low-power radio frequencies, and strong local transmissions can swamp them — car parks near airports, radio masts and some industrial sites are known trouble spots. If the fob works elsewhere but not in one particular place, this is why.

Water or impact damage. A fob through a washing machine sometimes recovers if dried thoroughly, and sometimes does not.

Loss of synchronisation. Fobs use rolling codes that change with each press. Pressing the button repeatedly out of range can, on some systems, move the fob's counter beyond what the car accepts. Manufacturers publish a resynchronisation procedure, usually involving the key in the ignition.

A flat car battery disables the receiver entirely, so nothing will respond.

Every car has a mechanical way in. Most fobs contain a hidden emergency key blade, and there is usually a physical lock on the driver's door, sometimes concealed behind a cover. Keyless-start cars also have a designated spot — often the steering column or a marked point in the console — to hold the fob so the car can read it passively.