starting problem with (reg nr : S 672 AHT) renault megane scenic diesel?

My S Reg Renault megane scenic diesel has a starting problem.the central lock key does not open infrared wise but only manually.the last 3 years i had to use the 4 digit pin code (78964) to start the engine but now it does not start.I was thinking it could be the crank sensor.I replaced it but still the same problem.would you please advise me to solve this problem ?

Abukhaled
February 2013