1998 Citroen Saxo 1.5 diesel immobilizer fault?

Hi all does anyone have fix for this immobilizer fault when I insert the ignition key and switch on both the red and green light come after a second or two I have tried punching in the correct code into the keypad but both the lights stay on and this prevents the car from starting ,,,, I found this fix on the internet which said turn the ignition on for half hour or so to see if this will clear the fault then switching off and removing the key and then reinserting the key and switching on I have tried this but it does not clear the fault so can anyone advise me what the problem is and a fix if one is available.
many thanks and best regards to all readers