Symptom
Immobiliser or security light flashing
A car outline with a key, a padlock, or a plain red dot that blinks. Slow blinking with the vehicle locked and the key away is normal — that is the system advertising itself to a thief. What matters is what it does when the key is present: continuing to flash, or staying lit, means the immobiliser has not recognised the key and will not release the engine. The car almost always cranks perfectly and either refuses to fire or starts and dies within a second or two, which is the signature that separates this from a fuel or spark problem.
Also described as: immobiliser light, immobilizer light, security light, anti-theft light, key light flashing, car with key symbol, red light blinking on dash, immobilizer or security light flashing, چراغ دزدگیر چشمک میزند, چراغ ایموبیلایزر, چراغ سوییچ چشمک, luz inmovilizador parpadea, testigo llave parpadeando, luz seguridad parpadea, luz inmovilizador parpadeando, لمبة الإيموبلايزر ترمش, علامة المفتاح تنور, إشارة الحماية ترمش
Some causes mean you should stop driving
4 of the 6 codes that produce this symptom indicate a fault where continuing to drive risks damage or is unsafe. If the symptom is sudden or severe, treat it as one of those until a scan says otherwise.
What this usually means
Slow blinking with the vehicle locked and the key away is normal — that is the system advertising itself to a thief, and it is supposed to do it. What matters is what the lamp does when the key is present. Continuing to flash, or staying lit, means the immobiliser has not recognised the key and will not release the engine.
The starting behaviour is the signature that separates this from a fuel or spark problem, and it is very specific. The car almost always cranks perfectly and either refuses to fire at all, or starts and dies within one to three seconds — German references give that window, and the mechanism explains it: the module permits the start and then decides the key was not authorised. An engine that plainly gets fuel and spark, runs for a moment and stops, is not a fuelling fault.
Try the spare key before anything else. It costs nothing and it splits the problem cleanly: if the spare works, the fault is in the first key — usually a transponder that has failed or a battery flat enough that the passive system cannot read it. If neither key works, the fault is in the vehicle — most often the reader coil around the ignition barrel, which corrodes, or its wiring.
Aftermarket alarms and immobilisers are a population of their own, and they are worth asking about directly because they are frequently fitted, rarely documented, and fail with age. A car that has an extra module spliced into the ignition circuit will not be diagnosed correctly from the manufacturer's wiring diagram.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Cranks normally, fires and dies after one to three seconds, every time | Immobiliser authorisation failing — the module permits the start and withdraws it | Watch the security lamp through the whole attempt, then try the spare key. Read immobiliser codes rather than engine ones. | Renew the key transponder, or repair the reader coil at the barrel. Re-learn the key to the vehicle afterwards. |
| Spare key works normally | The first key — a failed transponder, or a battery too flat for the passive system to read | Try the failing key again with a fresh battery, and try holding it against the start button or barrel where the vehicle supports emergency reading. | Renew the key battery, or have the key renewed and coded. Keep the working spare somewhere safe meanwhile. |
| Neither key works | The vehicle side — the reader coil around the barrel, its wiring, or the immobiliser module | Read immobiliser codes. Inspect the coil connector at the column, which corrodes and is a known failure point. | Repair or renew the coil or its wiring. Module replacement needs coding to the vehicle and to the keys. |
| Started after an aftermarket alarm or tracker was fitted, or with age | An aftermarket immobiliser cutting a circuit it should be releasing | Ask what has been fitted and find the module. It will not appear on the manufacturer's wiring diagram, which is why this is asked rather than measured. | Repair or remove the aftermarket unit properly, restoring the original wiring. Do not bypass a factory immobiliser to work around it. |
| Intermittent — works most days, refuses occasionally | A marginal read — a weak key battery, a damaged transponder, or a coil connection that moves | Note whether it correlates with cold, damp, or which key is used. Renew the key battery first as the cheapest variable. | Renew the key battery, then the transponder or coil. An intermittent immobiliser strands people and should not be left. |
| Slow blink with the car locked and the key indoors | Nothing — the system is advertising itself, which is its deterrent function | Confirm the blink stops and the car starts normally when the key is present. | No repair. |
-
Cranks normally, fires and dies after one to three seconds, every time
- Likely cause
- Immobiliser authorisation failing — the module permits the start and withdraws it
- How to confirm it
- Watch the security lamp through the whole attempt, then try the spare key. Read immobiliser codes rather than engine ones.
- What fixes it
- Renew the key transponder, or repair the reader coil at the barrel. Re-learn the key to the vehicle afterwards.
-
Spare key works normally
- Likely cause
- The first key — a failed transponder, or a battery too flat for the passive system to read
- How to confirm it
- Try the failing key again with a fresh battery, and try holding it against the start button or barrel where the vehicle supports emergency reading.
- What fixes it
- Renew the key battery, or have the key renewed and coded. Keep the working spare somewhere safe meanwhile.
-
Neither key works
- Likely cause
- The vehicle side — the reader coil around the barrel, its wiring, or the immobiliser module
- How to confirm it
- Read immobiliser codes. Inspect the coil connector at the column, which corrodes and is a known failure point.
- What fixes it
- Repair or renew the coil or its wiring. Module replacement needs coding to the vehicle and to the keys.
-
Started after an aftermarket alarm or tracker was fitted, or with age
- Likely cause
- An aftermarket immobiliser cutting a circuit it should be releasing
- How to confirm it
- Ask what has been fitted and find the module. It will not appear on the manufacturer's wiring diagram, which is why this is asked rather than measured.
- What fixes it
- Repair or remove the aftermarket unit properly, restoring the original wiring. Do not bypass a factory immobiliser to work around it.
-
Intermittent — works most days, refuses occasionally
- Likely cause
- A marginal read — a weak key battery, a damaged transponder, or a coil connection that moves
- How to confirm it
- Note whether it correlates with cold, damp, or which key is used. Renew the key battery first as the cheapest variable.
- What fixes it
- Renew the key battery, then the transponder or coil. An intermittent immobiliser strands people and should not be left.
-
Slow blink with the car locked and the key indoors
- Likely cause
- Nothing — the system is advertising itself, which is its deterrent function
- How to confirm it
- Confirm the blink stops and the car starts normally when the key is present.
- What fixes it
- No repair.
Codes that produce this symptom (6)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0513 Incorrect Immobiliser Key Typical of this code Stop driving High
- P0633 Immobiliser Key Not Programmed — Engine Control Module Typical of this code Stop driving High
- P0648 Immobiliser Lamp Control Circuit Typical of this code Low
- U0167 Lost Communication With Vehicle Immobiliser Control Module Typical of this code Stop driving High
- U0326 Software Incompatibility With Vehicle Immobiliser Control Module Stop driving High
- U0426 Invalid Data Received From Vehicle Immobiliser Control Module High
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Others with this symptom
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about Immobiliser or security light flashing.