How long ago did you change your cap and do you still encounter the same proble? This matter of 'check engine' is very common nowadays. My 540i BM show the same sign and went off by itself without doind anything to it. 'Check Brake lining' light also showed up on the BM even after fininshing brake job.
I went through motor vehicle inspection in NJ with 'Check brake lining' sign & the car passed inspection.
Most cars nowadays will show you 'Check engine ' which most mechanics are making money of the owners.