Take it to the dealer and get them to check the codes. It can be simple, like a sensor, or expensive, depending on what the problem is.

You don't give you location so it is not clear to me what your warranty terms are, but in most countries there is an emissions system warranty that runs longer than the vehicle warranty. If so, it may be covered by the emissions warranty even if the vehicle is out of the full warranty coverage period.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue