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Warning lights and alertsUrgency: Garage soon

Immobiliser warning light (key or padlock) on: causes and the faults that trigger it

The immobiliser is the anti-theft system that checks the code on the key's chip and, if it does not recognise it, blocks the injection or ignition: that is why the car does not start and a warning light with a key or a padlock may come on, or the message "key not detected" may appear. The first things the sources recommend are trying the spare key, changing the fob battery and checking the car battery; if it starts with the spare, the fault is in the original key or in its reader. If none of that works, you need an automotive electronics specialist, because the fix is usually reprogramming the key or replacing the antenna or another component.[1][2][3]

Colour: red. Symbol: a car with a key, or a key or a padlock, sometimes with the message "key not detected".

2 documented faults cause it in 2 cars:

    What it means

    The immobiliser is the electronic anti-theft system that blocks starting when the car does not recognise the chip in the key. Its warning light usually shows a key or a padlock, or a "key not detected" message, and may flash. Usual causes are the key fob battery, a damaged chip, the reader antenna, the car battery or the control unit.

    General explanation from MotorFiable. How it shows up in each engine, with sources, is in the list of faults.

    What to do now

    Garage soon

    Try the spare key, replace the key fob battery if it responds poorly, keep other electronic devices away and check the car battery. If it still will not start, ask a garage with electronic diagnostics for help.

    12 sourcesHow we work

    Most common causes

    From most to least likely according to the sources. Under each cause, the part and the related OBD codes.

    1. 1. Low or flat car battery

      Common cause

      Diariomotor lists a flat battery as the most common fault when a car does not start, with symptoms such as very dim interior lights or a fob that does not work; RACE includes the car battery among the points checked when the immobiliser is suspected.[1][3]

      Faults with with the 12 V battery

    2. 2. Flat fob battery or worn key

      Common cause

      RACE and Autopista cite a flat fob battery and worn keys among the reasons the car does not recognise the key. Autopista recommends checking the key battery before anything else, and Autofácil points out that if the fob has no battery, the car will not start.[1][2][4]

    3. 3. Damaged or decoded key chip (transponder)

      Possible cause

      Keys contain a chip that the car reads when you turn the ignition. RACE includes a damaged chip among the causes and says the immobiliser can prevent starting when the key has been decoded; Diariomotor explains that if it starts with the spare key, the problem lies with the original or its reader.[1][3][5]

    4. 4. Interference or frequency jammers

      Possible cause

      RACE advises moving other electronic devices away and points out that frequency jammers, common near official buildings, stop the fob and keyless start from working. Autofácil mentions the same jammers.[1][4][5][6]

    5. 5. Faulty immobiliser reader antenna

      Possible cause

      RACE cites the reader antenna among the points that can fail and among the usual garage solutions, along with reprogramming or pairing the key and repairing or replacing the fob.[1]

    6. 6. ECU or ECU software fault

      Uncommon

      RACE and Autopista include electronic problems with the ECU among the causes, and Autopista adds software alterations.[1][2]

      Faults with with the control units

    7. 7. Damaged wiring

      Uncommon

      RACE mentions the wiring among the points a technician checks when the immobiliser will not let the car start.[1]

      Faults with with the electrical wiring

    8. 8. Other starting faults that get confused with the immobiliser

      Possible cause

      Not every car that fails to start has the immobiliser locked. Diariomotor distinguishes the typical "clack" of a worn starter motor, fuel problems, glow plugs on diesels and fuses; Autofácil also cites the starter motor, the high-pressure pump and the injectors.[3][4]

      Faults with with the alternator and starter motorFaults with with the fuel pump

    Depending on what you notice

    The immobiliser won't let the car start

    RACE lists the symptoms that it may be acting: the car unlocks but does not start, "key not detected" appears, the key or padlock warning light flashes, the starter motor turns without the engine firing, it starts and stalls immediately, the key works intermittently or the copy does start it. Autopista advises starting with the key battery and, if that does not help, going to an electronics specialist.[1][2]

    The key turns but the car won't start

    Diariomotor explains that if the immobiliser fails, there are no signs of a low battery or anything else and the car simply does not react: the test is to use the spare key, and if it starts, the fault is in the original key or its reader. If what you hear is a "clack", that points more to the starter motor, and a flat battery remains the most common fault.[3]

    How to start a car with the immobiliser active

    No source describes a way of bypassing the immobiliser. What RACE records is to try the second key, change the fob battery, move other electronic devices away and check the battery, and at the garage to reprogram the key or replace the antenna. With keyless start, if the car does not detect the fob, RACE says to hold it close to the start button or the emergency slot, whose position depends on the make.[1][6]

    The key or padlock warning light is flashing

    RACE includes a flashing key or padlock warning light among the symptoms that the immobiliser may be acting. With the car switched off, however, it can be normal: Autofácil notes for Toyota a security indicator that flashes after the key is removed to show that the system is working.[1][7]

    Key not detected message on a car with keyless start

    RACE explains that the fob sends a short-range signal and that, if its battery runs flat or there are frequency jammers, the car may fail to recognise it; in that case you can hold the fob close to the button or the emergency slot, or use the physical key inside the fob.[5][6]

    After several attempts with a key the car does not recognise

    Wikipedia notes, in general terms, that if the code is not recognised the ECU does not allow fuel through or let the engine fire, and that on some systems trying to use an unauthorised key triggers a timed starting lockout. No Spanish-language source confirms this for specific makes.[8]

    What to check first

    1. Look for a key or padlock warning light, or a "key not detected" message, and how it behaves (steady or flashing).[1]
    2. Try the second key: if the car starts with it, the fault is in the original key or its reader.[1][3]
    3. Change the fob battery if it responds poorly or intermittently; Autopista lists it as the first thing to check.[1][2]
    4. Move other electronic devices away and avoid areas with frequency jammers, which can stop the car recognising the fob.[1][5]
    5. Check the car battery: very weak interior lights or a fob that does not work give it away, and with a multimeter it should read 12 V or more.[1][3]
    6. If it still will not start, consult the manual and go to an automotive electronics specialist with diagnostic equipment.[1][2]

    Can I keep driving?

    Urgency: Garage soon

    With the immobiliser acting, the car does not start, so the question is not whether you can drive it but how to get it going again. The sources recommend ruling out the simple things first (spare key, fob battery, car battery) and, if that is not enough, going to an electronics specialist with diagnostic equipment. If you only see the warning light flashing with the car switched off and it starts normally, it may be the system's normal operation, as on Toyotas according to Autofácil.[1][2][7]

    How much it costs to fix

    Indicative prices published by the sources cited; they depend on the car and the garage.

    RepairPrice
    Copying a key with a transponder (indicative RACE price; it has to be programmed)[9]€80
    Copying a key with a remote control (indicative RACE price)[9]€100 – €120
    Copying a key card (indicative RACE price)[9]€150 – €180
    Copying a hands-free fob (indicative RACE price)[9]€300 – €380
    Getting another set of keys if you lose the originals (according to RACE, it depends on the type of key, the make and the model)[10]€30 – €300
    Replacing the car battery (average cost given by Autofácil; it does not make clear whether labour is included)[4]€80 – €90

    Faults that cause it

    Suspension, brake, electronics and other faults on specific cars, from most to least important.

    1. Source for the symptom in the Mercedes-Benz Clase C W203[11]

    2. Source for the symptom in the Seat León III[12]

    Immobiliser light: causes in your car

    2 cars have several documented faults that cause this symptom. Choose yours to see only those that affect it, by engine and by year.

    Mercedes-Benz 1
    Seat 1

    Frequently asked questions

    Can the immobiliser be bypassed or deactivated?

    None of the sources consulted describes how to disable it. The solutions RACE records are those of the garage: reprogramming or pairing the authorised key, repairing or replacing the fob and replacing the reader antenna. Autopista adds that the system's code is regenerated on each use, which makes duplicating the key difficult.[1][2]

    How much does it cost to copy or replace a key with a chip?

    RACE gives around €80 to copy a key with a transponder, between €100 and €120 if it has a remote control, €150-180 for a card and €300-380 for a hands-free fob. If you lose the keys, getting another set can cost between €30 and €300 depending on the type, make and model.[9][10]

    How does the immobiliser work?

    The chip in the key sends a code to the car's ECU, which compares it with the one it has programmed. If it matches, the engine starts; if not, the ECU blocks the injection or ignition.[1][2][8]

    Could it be the battery and not the immobiliser?

    Yes. Diariomotor lists a flat battery as the most common fault when a car does not start, and RACE includes the battery among the points checked before blaming the immobiliser. Check that the interior lights and the fob respond normally.[1][3]

    Why doesn't my keyless-start car detect the fob?

    RACE gives two reasons: a flat fob battery and frequency jammers, found near official buildings. In both cases you can hold the fob close to the start button or the emergency slot, or use the built-in physical key.[5][6]

    What do I do if I lose the car key?

    RACE advises keeping a spare set, notifying your insurer or roadside assistance, contacting the manufacturer, which on some makes offers a recovery and copying service, or going to an automotive locksmith for electronic keys.[10]

    Other symptoms: warning lights and alerts

    All symptoms

    Has this happened to you? Tell us which fault it was, at how many kilometres and how much you paid: it helps other owners know what to expect. Report your fault.

    Sources

    Every fact on this page links to its source. If you find an error, let us know.

    1. ¿Qué es el inmovilizador de un coche?RACE · Motoring press · published 25 September 2026 · consulted 4 October 2026 · in Spanish
    2. Inmovilizador de coche antirrobo: ¿Es útil? ¿Cómo funciona?Autopista · Motoring press · published 21 July 2025 · consulted 4 October 2026 · in Spanish
    3. Mi coche no arranca: ¿qué avería puede tener?Diariomotor · Motoring press · published 6 August 2020 · consulted 4 October 2026 · in Spanish
    4. ¿Por qué mi coche no arranca?Autofácil · Motoring press · published 24 September 2022 · consulted 4 October 2026 · in Spanish
    5. ¿Tu coche no arranca? Te contamos cuáles son las causas más comunesRACE · Motoring press · published 23 May 2023 · consulted 4 October 2026 · in Spanish
    6. ¿En qué consiste el arranque sin llave en un coche?RACE · Motoring press · published 26 July 2023 · consulted 4 October 2026 · in Spanish
    7. ¿Qué significan los testigos de los coches Toyota?Autofácil · Motoring press · published 5 July 2022 · consulted 4 October 2026 · in Spanish
    8. ImmobiliserWikipedia · Other source · consulted 4 October 2026
    9. ¿Cuánto cuesta hacer una copia de las llaves del coche?RACE · Motoring press · published 26 April 2023 · consulted 4 October 2026 · in Spanish
    10. ¿Qué hacer si pierdes las llaves del coche?RACE · Motoring press · published 30 April 2021 · consulted 4 October 2026 · in Spanish
    11. Avis Mercedes Classe C (W203, 2000-2007): opiniones de propietariosFiches-auto.fr · Motoring press · consulted 27 September 2026 · in French
    12. Problemas Seat Leon 3 | Fallos y averías más frecuentesTotal Renting · Other source · consulted 26 September 2026 · in Spanish