Your car key fob has stopped working: how to diagnose it

The five-minute diagnosis

Work through these in order. Most fobs are fixed before step four.

1. Change the battery. It is the cause more often than everything else on this list combined. A weak cell fails intermittently first — works from a step away, not from across the car park — then fails completely, usually on a cold morning.
2. Try the spare fob. This single test splits the problem in half. If the spare works, the fault is inside the first fob. If neither works, look at the car.
3. Check the car's own battery. A vehicle battery low enough to upset the body electronics makes perfectly good fobs look broken. Dim interior lights, a slow crank or a dashboard full of warnings are the clue.
4. Check for damage. Fobs get dropped on concrete, sat on, and put through washing machines. A cracked circuit board or a corroded contact from a spilled drink can leave the outside of the case looking untouched.
5. Consider where you are. If it only misbehaves in one car park or one street, that is interference, not the fob. Radio noise around some buildings and structures is genuinely bad. If it misbehaves everywhere, it is the fob.
6. Then think about pairing. A fob that has genuinely lost its pairing to the vehicle needs reprogramming. That is the point where equipment is needed and where somebody comes out to you.

Getting into the car right now

• Find the emergency blade. Almost every fob hides one. There is usually a small catch, button or slider on the side or the back; the blade pulls straight out.
• Find the keyhole. Most cars have one on the driver's door. On many it is concealed under a small plastic cap on the handle, which pops off with a fingernail or by pulling the handle out and sliding the cover.
• Expect the alarm. Opening a locked door mechanically can set it off. That is normal. Getting in and starting the car usually stops it.
• If your car is push-to-start, use the backup start method: fob held against the start button, or against the marked spot on the column or console. The manual has the exact position.

Changing the battery without breaking the fob

1. Take the emergency blade out first — on many fobs, removing it releases the case.
2. Find the seam. Use a coin or a plastic pry tool along it, not a screwdriver.
3. Note which way round the old cell sits before you lift it out. Photograph it if you are not sure.
4. Handle the new cell by the edges. Fingerprints on the flat faces cause poor contact.
5. Do not touch or flex the circuit board. Press the case back together until it clicks all the way round.

What not to do

• Do not lever the case apart with a metal screwdriver. The seams are shallow and the board sits directly behind them. This is how a dead battery becomes a dead fob.
• Do not press buttons repeatedly hoping it re-pairs. It will not, and on some vehicles a run of failed attempts pushes the system into a lockout that has to time out before anything can be programmed at all.
• Do not buy a replacement fob before you know the fault. If the vehicle battery, a fuse or the receiver is the problem, a new fob changes nothing and you now own two of them.
• Do not try to dry a soaked fob with heat. A hair dryer or an oven warps the case and cooks the board. If it went in water, take the battery out immediately and let it dry in air.
• Do not assume a non-working fob means a non-working car. The blade and the backup start method exist precisely for this.

When it is a call-a-pro moment

• New battery, still nothing, and the spare fob works — the fob itself has failed.
• Neither fob works and the car's battery is healthy.
• You have lost every fob. That is an all-keys-lost job — see what to do when you have lost every car key.
• The fob unlocks the doors but the car will not start, or starts and dies within a second. That is the immobiliser refusing the key.
• You want a spare made while you still hold a working one, which is by far the easier version of this job.

What a locksmith actually does

1. Confirms the vehicle is yours — photo identification plus registration, title or an insurance card in your name for that plate.
2. Tests whether the fob is transmitting at all, which separates a dead fob from a vehicle that has stopped listening.
3. Pairs a new or existing fob to the vehicle through its diagnostic port.
4. Cuts and fits the emergency blade where the new fob needs one.
5. Erases lost or stolen fobs from the vehicle's memory if you ask — important if one has gone missing.
6. Tests locking, unlocking, the trunk release and starting, with you there. That is our key fob programming work, done at the vehicle.