If it was only a temporary limp mode that went away after a restart then it could be a VANOS problem. My N54 135i would sometimes go into limp mode temporarily after WOT acceleration as you described and when I finally took it in for service the third time it happened the dealer identified it as a VANOS solenoid related problem. The only mod I had was dual cone intakes.
Dackel usually has great advice but in this case I have to disagree. A yellow engine light could point to a serious problem - I would not ignore it. Definitely get your car to a service center as soon as possible. Remove any aftermarket engine mods before you head to the dealer, though.
2011 1M | Valencia Orange | #655 of 740
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