--------------------------------------------------------------------------------

i noticed that the checkengine light goes on, on 1st gear at 2000rpm and goes off at 3000rpm, and when i shift to 2nd gear it goes on at 1800rpm and goes off at 2200rpm. on 3rd gear it appears sometimes and sometimes but not on 4th or 5th and not when i shift back to 2nd.why? it's driving me crasy!!!

 

probably induced by engine vibrations.

check your electrical connections.

 

You probably should also dump the CEL codes to see if anything needs fixing. On an S5 its really easy to do.

 

Pull the codes, how-to:

http://www.banzai-racing.com/FD&S5_error_codes.htm