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You only have a set time to do it, there may be some advice out their on a Renault forum but I did find this.
Key no longer turns off car immobliser (1998, RT)
As an alternative to buying a new key, from Jamie Ford: I have found a solution, It can be used all the time but does take a few moments to perfect. What the owner needs to do is find out the 4 Digit code for the immobiliser from the dealer. They usually complain and say this number does not exist but it does. Depending on the dealer there may be a charge for this number (no more than £30). When you have the number, you turn on the ignition. The red light on the dash should flash rapidly, press in the lock button on the dash and the light should slow down. You need to count the number of flashes to correspond with the first digit of the code, then release the button pressing it in again for the second number and so on. When all numbers have been entered in this way the button should be released and the light will stay illuminated for about 10 seconds - when the light goes out the immobiliser is turned off and you can start the car.... It sounds a little long winded but when you get the hang of it it takes about 30 sec to complete and saves the owner about 150 quid on a new key....
Source(s):
http://www.bba-reman.com/content.aspx?content=Test_for_Renault_Key_Immobiliser_failures_emergency_recode
to download program to generate the 4-digit code.
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