non starter / immobiliser

[gasman15]

its not a 172 but at work we have a clio 1.2 16v and we changed the spark plugs on the car but it wont start. it turns over for like 3 seconds and then stops like the immobiliser as come in

you have to turn the ignition off and try again but the same happens.if you dont turn the key for like 3 second and then try and strart it nothing happens at all

any idea's?

thanks matt