is it the renault immobiliser?

few days ago my car wouldnt start,renault megane 98 r reg,it turned over the same as if u turn over with immobiliser on,it makes a different noise when trying 2 start,anyway,called aa out,they said it was crankshaft sensor,put new 1 in,still not starting,does any 1 know of any problems/solutions 2 this,or has any 1 had same problem...?
ty in advance
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