car dose not start

[puneet]

i have a SLk 2003 and every now and then i try to start the ignition switch a red&green light flash continuously on my rear view mirror and the engine wont start.

at times if i leave it for few hours and try again it starts and at other times it takes overnight.

what could have gone wrong

please advice

[raghavdxb]

check starter relay, if not crank sensor is gone.