What do I do if my car key fob stops working?

How to tell a flat fob battery from a car battery problem, the emergency methods almost every car has for getting in and starting, and when this becomes a dealer job.

Difficulty: beginner
Time: 20 min
Read: 5 min
Safety: caution

Short answer

Try the spare fob first — if that works, the problem is the fob, and it is almost always the battery. Nearly every fob hides a mechanical key blade released by a small catch, which opens the driver's door, and nearly every push-button car has a place to hold a dead fob so it can still be read — usually against the start button or a marked slot. Both are in the handbook. If both fobs are dead at once, suspect the car's own battery or radio interference rather than the fobs.

A modern key does two unrelated jobs with two unrelated systems, and knowing which one has failed saves a lot of trouble. The remote locking is an ordinary radio transmitter and needs battery power to send anything. The immobiliser is a passive transponder chip in the fob that draws its power from a coil in the car, needs no battery at all, and answers a cryptographic challenge before the engine will start. That is why a fob with a completely dead battery will still start the car if you hold it in the right place — the car is powering the chip directly — while the buttons do nothing at all.

Safety

If a fob fails while you are away from home, do not leave a car unlocked and unattended with anything visible in it — insurers routinely decline claims for theft from an unsecured vehicle. Take care prising a fob apart: the batteries are small lithium cells that are extremely dangerous if swallowed by a child or a pet, so keep the old one away from them and dispose of it in a battery recycling point rather than a bin.

Step by step

Try the spare fob.This is the fastest diagnosis there is. If the spare works normally, the first fob is the problem. If neither works, the fault is in the car — its battery, its receiver, or interference. Find the spare before you need it, and check it works occasionally; spares left in a drawer for five years have flat batteries too.
Stand right next to the car and try again.Radio interference genuinely defeats fobs. Car parks near airports, some hospital sites, transmitter masts and even a nearby vehicle with a fault can swamp the frequency. Moving a few metres or holding the fob against the door handle often works when it will not from a distance.
Find the mechanical key blade.Almost every fob, including slim smart keys with no visible keyhole, contains a metal blade released by a small slider or button on the side or back. It opens the driver's door — often through a lock cylinder hidden under a cap on the handle that prises off. The handbook shows exactly where.
Expect the alarm to sound when you open it that way.Opening the door with the blade while the car is armed usually triggers the alarm on many vehicles, because the car has not seen a valid disarm signal. It stops once the car reads the transponder — getting in and pressing start is normally the fix. Know this before it happens in a supermarket car park.
Use the emergency start position.Push-button cars have a defined backup: holding the fob directly against the start button, placing it in a marked slot in the centre console or cupholder, or presenting it to a specific point on the steering column. It works because the car's coil powers the transponder chip directly. It is in the handbook and it is worth reading before you need it.
Change the battery.Most fobs come apart with a coin or a small screwdriver in a moulded slot, sometimes after removing a screw. Note the battery type stamped on the cell — commonly CR2032 or CR2025 — and which way up it sits before removing it. Fit a fresh branded cell and handle it by the edges, because skin oils shorten their life.
Check for water damage if a new battery does not fix it.Fobs that have been through a wash cycle or spent an afternoon in a coat pocket in the rain corrode internally. Sometimes drying it out thoroughly for a day recovers it; often it does not, and it needs replacing and programming.
Suspect the car if both fobs fail together.A car battery that is very low interferes with keyless entry and immobiliser systems long before it fails to start the engine. A dead car battery, a blown fuse feeding the receiver, or a failed receiver module all present as "my key has stopped working", and the fix is on the car rather than in your pocket.

Common mistakes

Not knowing where the mechanical key blade is until you need it — It is a thirty-second thing to find on a dry afternoon and a miserable thing to hunt for in the rain with shopping. The same goes for the emergency start position.
Assuming a fob that has stopped working needs replacing — The overwhelming majority are a coin cell costing a pound or two. Replacement fobs need programming to the car and cost a great deal more, so try the battery first.

If it doesn't work

Buttons do nothing but the car starts when you get in

Cause: Flat fob battery — the transponder still works passively — Fix: Exactly the expected pattern. Change the fob battery and the remote locking returns.

Car unlocks but will not start, with a key symbol on the dash

Cause: The immobiliser cannot read the transponder — Fix: Try the emergency hold position from the handbook. If that fails, the transponder or the car's reader has failed, and it needs a dealer or an auto locksmith with the right equipment.

Works from one metre, nothing from ten

Cause: Weak fob battery, or a partly damaged aerial in the fob — Fix: Change the battery first. Reduced range is the classic early warning of a fob battery going flat, weeks before it stops entirely.

Both fobs stopped working on the same day

Cause: The car, not the fobs — battery, fuse or receiver — Fix: Check whether the car starts with the emergency method and whether the interior lights are strong. A very low car battery is the most common cause.

Fob works everywhere except at home or at work

Cause: Local radio interference — Fix: Use the mechanical blade there. Common near transmitters and some commercial premises, and there is nothing wrong with the car.

New battery fitted and now nothing works at all

Cause: Battery in the wrong way round, a contact bent, or the case not fully closed — Fix: Open it again and check the orientation against the marking in the housing, and that the small contacts are touching the cell.

Questions people ask

Will a dead fob battery stop the car starting?

On most cars, no — the immobiliser transponder is passive and draws power from the car, so the emergency hold position still works. On a few designs it does prevent starting, which is exactly why the handbook's backup procedure is worth reading now rather than later.

Can I program a replacement fob myself?

Some older cars had an owner procedure in the handbook. Almost all modern cars require dealer or auto locksmith equipment with security access, because that is the point of the system. Beware very cheap fobs sold online that cannot be programmed to your car.

Why does my fob only work when I hold it against my head?

It is not a myth. Your body acts as a crude reflector and can extend the range slightly. It is also a clear sign the battery is nearly flat, so treat it as a warning rather than a technique.