S-430 Stopped @ red light and Check engine light came on
 

Stopped at red light and check engine light came on. I tried to start a couple of times but all I got a was a clicking sound. However after a few minutes - it started and the CEL stayed on but the car was running fine.

A friend told me that this might be a mis-fire and helped reset the CEL. It ran fine for the last few months and then yesterday, it had the same issue in the garage trying a cold start. Started after I tried again in 10 min.

Please advise how best to resolve this - the dealership will charge an arm/leg. Is there a cheaper smarter way to diagnose the problem?