Yes, a dead battery in a key fob stops a keyless push-button car starting normally, but a car with a key blade in the ignition keeps starting.
The transponder chip inside every car key answers the car without battery power at close range. Key blade cars lose only the remote locking, while keyless cars need the fob at a backup position that differs by make. Bright dashboard lights separate a flat key fob battery from a flat car battery. A key fob battery lasts 1 to 4 years, and a flat one means the car does not unlock.
A backup start leaves the dead key fob battery in place, and how far a keyless car drives without the key depends on fuel. Drivers replace car key batteries at home, and faults that survive a new cell need key fob repairs. A fix costs £2 to £400.
The list below gives four key takeaways on a dead key fob battery.
- A dead key fob battery stops the normal start on keyless push-button cars only.
- The transponder chip starts the car without battery power at the backup position.
- The car battery is at fault when the dashboard lights dim and the starter clicks.
- A CR2032 coin cell fits most key fobs.
Which cars start with a dead fob battery?
Cars with a key blade that turns in the ignition still start with a dead key fob battery, and keyless cars start only from the backup position.
The ignition barrel powers the transponder chip by induction, so the immobiliser reads the chip and only the remote locking stops.
A keyless car sends a low-frequency search signal, and the fob answers with power from the key fob battery. A flat cell leaves the car without a reply, so the dashboard shows “Key not detected” and the immobiliser stays armed.
How do keyless cars start on dead fobs?
To start a keyless car with a dead key fob battery, hold the fob against the backup position and press the start button with the brake pedal down. The list below gives the four steps.
- Open the driver’s door with the emergency key blade hidden inside the fob.
- Find the backup position for the make.
- Press the brake pedal, or the clutch on a manual, and hold the fob against the backup position.
- Press the start button with the fob still in place, and fit a new key fob battery before the next drive.
Where is the fob backup position by make?
The start button, the steering column or a marked slot holds the backup position for the key fob, depending on the make. The table below lists the backup position for five groups of makes.
| Make | Backup position on many models |
|---|---|
| Toyota, Lexus, Nissan, Kia, Hyundai, Honda | Hold the fob against the start button |
| Volkswagen, Audi, Seat, Skoda | Hold the fob to the steering column mark |
| BMW, Mini | Hold the fob to the steering column mark, or use the older slot |
| Ford | Place the fob in the cup holder or centre console slot |
| Mercedes-Benz | Place the fob in the marked cup holder spot |
The owner’s handbook shows the backup position for the key fob with a diagram.
Which is flat, the fob or car battery?
The key fob battery is flat when the dashboard lights stay bright, and the car battery is flat when the lights dim and the starter clicks. The table below compares four signs of each fault.
| Sign | Flat key fob battery | Flat car battery |
|---|---|---|
| Dashboard lights | Come on normally | Dim, flicker or stay off |
| Dashboard message | "Key not detected" or "Key battery low" | None in most cases |
| Starter motor | Does not turn at all | Clicks or turns slowly |
| Backup fob position | Starts the car | Makes no difference |
Both faults stop the normal start and weaken the remote locking. A flat key fob battery needs a new coin cell, while a flat car battery needs a jump start or a charge.
How long does a key fob battery last?
A key fob battery lasts between 1 and 4 years, and keyless fobs sit at the shorter end because the fob answers the car constantly.
Most fobs take a 3-volt coin cell, including the CR2032, CR2025, CR2016 and CR1620. The list below gives the four warning signs of a failing key fob battery.
- The dashboard shows a “Key battery low” message.
- The keyless car asks for the fob to move closer before the engine starts.
- The remote locking works only within 1 to 2 metres of the car.
- The buttons need two or three presses before the car responds.
A flat key fob battery is one of four reasons a car does not unlock, alongside a lost pairing, a seized door lock and a worn key blade.
What happens after a dead fob backup start?
The engine runs normally after a backup start, and the dead key fob battery still needs a new cell. The five questions below cover the next steps.
Frequently asked questions
How far do keyless cars drive without keys?
A keyless car drives until the engine stops or the fuel runs out, because the car checks the key fob only at the start. The fuel or the charge on board sets how far a keyless car drives without the key.
How long does a low fob battery last?
A key fob battery lasts 2 to 8 weeks after the first low battery warning, and keyless fobs sit at the shorter end.
A spare CR2032 coin cell in the glovebox makes the swap a five minute roadside job.
Can drivers replace car key batteries at home?
Yes, drivers replace car key batteries at home with a small flat blade in about five minutes.
The five steps to replace car key batteries at home open the key fob case at the seam and fit the new cell the same way up.
Which key fob repairs fix a failing fob?
Contact, board and pairing repairs fix most key fobs that ignore a new battery. The list below gives the four causes, most likely first.
- A wrong-size, upside-down or corroded cell contact breaks the circuit, and a home check fixes the fault.
- A water-damaged or cracked circuit board stops the signal and needs testing equipment.
- A lost pairing stops the car recognising the fob and needs reprogramming.
- A car fault in the aerial or the immobiliser ignores a healthy fob.
ADC Locksmith Liverpool tests the fob and the car at the roadside, and key fob repairs cost less than a new fob.
What does fixing a dead key fob cost?
A dead key fob costs £2 to £5 to fix when a new coin cell solves the fault. The table below compares five typical UK prices for fixing a dead key fob.
| Fix | Typical UK price |
|---|---|
| New coin cell at home | £2 to £5 |
| Key fob repair | £40 to £80 |
| Key fob reprogramming | £60 to £120 |
| New fob from a mobile auto locksmith | £120 to £250 |
| New fob from a main dealer | £200 to £400 or more |
The make, the model year and the key type set the final price. A key blade remote costs less to replace than a keyless push-button fob.
Based in Liverpool and still stuck?
ADC Locksmith Liverpool repairs, programs and replaces car keys at the roadside across Liverpool and Merseyside, usually on the same day. Craig Evans holds SERMI approval and a 5.0 rating from 341 Google reviews. A call to 07731 544220 brings a fixed price before Craig travels.
Call 07731 544220