Always work to diagnose the problem before throwing parts at it.

While the Lambda sensors are 02 sensors, the code referring to them doesn't mean they are necessarily bad. It means they are reading out of range. That could be because they are bad, or because something else is causing a problem that causes them to read out of the expected range. A clue is that both are bad. You wouldn't expect both to fail at the same time, so it is worth looking for the root cause.

Once you have the codes, you follow a test plan to determine what the cause of the codes and problem is. Without a test plan, reading the codes tells you very little.

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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