warning light guide
Immobiliser (Security) Warning Light
Your car does not recognise the security code from your key or key fob. The engine will refuse to start or will cut out after two seconds to prevent theft.
What it looks like: It appears as a key, a padlock, or a car outline with a key inside, illuminated in red or amber. A slow pulse when parked is normal, but rapid flashing or a steady light with the ignition on signals a fault.
Updated 9 November 2026
Can I keep driving?
Action required
Immobiliser fault detected — engine may not restart
Your car is currently safe to drive home or to a garage, but DO NOT turn off the engine until you are in a safe location. The car may lock you out and refuse to restart once it is switched off.
What to do right now
- 1If the engine is running, keep it running until you are home or at a garage - once it is switched off, it may not restart.
- 2If the car will not start, switch off the ignition, take the key out and wait one minute before trying again.
- 3On a keyless car, hold the key fob against the start button or the marked spot on the steering column, then try again.
- 4Still no luck? Try your spare key - if it starts the car, the problem is your main key, not the car.
Likely causes
Most likely first.
- Likely
Flat key fob battery
A weak or dead coin battery prevents the key fob from broadcasting its wireless security signal to the car aerial. The car assumes an unauthorised entry attempt and blocks ignition.
- Possible
Damaged key transponder chip
Dropping the key or exposing it to water can crack the internal transponder or break its circuit board. The immobiliser reader cannot detect the programmed digital code.
- Possible
Weak 12-volt car battery
Low system voltage causes electronic control modules to drop communication during startup. The immobiliser module fails to authenticate the key before the starter motor engages.
- Rare
Faulty aerial reader ring or wiring
The coil antenna surrounding the ignition barrel or hidden behind the dashboard can degrade or lose power. Without it, the car cannot read even a perfectly healthy key.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Use emergency keyless start position
On push-button cars, hold the flat end of the fob against the start button or the marked spot on the steering column. This powers the transponder chip directly through induction even if the fob battery is dead.
- 2
Test with your spare key
Switch to your second programmed key to start the car. If the spare works immediately, the issue is isolated to your primary key fob rather than the vehicle electronics.
- 3
Replace the key fob battery
Prise open the key casing using a coin or flat screwdriver and fit a fresh CR2032 or CR2025 battery. Ensure the positive side faces the correct way and inspect the metal contact tabs for corrosion.
- 4
Check 12-volt vehicle battery health
A healthy 12-volt car battery reads around 12.6 volts at rest. If the battery rests below 12.0 volts or dips heavily during cranking, charge it or jump-start the car to see if the security light clears.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. Prices vary depending on whether the car uses a basic mechanical key with a transponder or a high-security proximity smart key requiring main dealer coding software.
Key fob battery replacement
- Do it yourself
- €3–8£3–7 in the UK
- At a garage
- €10–25£8–20 in the UK
New key supply, cutting, and electronic programming
- Do it yourself
- Garage job
- At a garage
- €140–380£120–330 in the UK
Immobiliser reader aerial or antenna replacement
- Do it yourself
- Garage job
- At a garage
- €160–420£140–360 in the UK
When to go to a garage
- The car refuses to start with both the main key and the spare key.
- A fresh key fob battery fails to stop the warning light flashing rapidly on the dashboard.
- The engine starts, runs for two seconds, and cuts out repeatedly.
- The dashboard displays a key unregistered or anti-theft defective message alongside the light.
Questions drivers ask
Can I keep driving if the immobiliser light comes on while the engine is running?
Most cars will not shut the engine off while driving at speed for safety reasons, but you must not turn the engine off until you reach a safe location or a garage. Once you switch off the ignition, the immobiliser will re-arm and will almost certainly prevent the car from restarting. Drive straight to a repair centre or your home.
Why is the immobiliser light flashing when my car is parked and locked?
A slow flash every two to three seconds when the engine is off and the vehicle is locked is entirely normal. It indicates that the anti-theft system is armed and actively protecting the car. It only indicates a fault if it flashes rapidly or stays continuously lit while you are trying to switch on the ignition.
Will disconnecting the 12-volt battery reset the immobiliser?
Disconnecting the battery rarely fixes an immobiliser fault because security codes are stored in non-volatile vehicle memory that persists without power. In fact, cutting battery power on some modern vehicles can desynchronise convenience settings or lock the audio unit. If low voltage caused a temporary communication glitch, fully recharging the battery is the correct step.
Does an illuminated immobiliser light cause an NCT or MOT fail?
The immobiliser warning light itself is not a listed fail item on the NCT or MOT. However, if the fault stops the tester starting the engine for the emissions or brake tests, the test cannot be completed. If it also turns on the engine management light, that is an automatic fail on the UK MOT; on the Irish NCT, engine fault codes on their own give an Advisory Pass.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.