Bad Cat or O2 Sensors?
 

2002 X5 E53 200K with OEM Exhaust and original Cat and no upgrades: Getting error codes on both pre-Cat and post-Cat Oxy Sensors. What are the chances that both are bad? Or is this a sign that the Cat is bad? If Cat and/or Sensors need replacing, which replacement brands do you recommend? Thanks!

What are the codes? How old are the plugs? Any recent maintenance?

From Bavarian Autosport Code Reader:
C6 - Cat Efficiency Cyl #1 - 3
E3 - Oxy Sensor Adaption Limit #1 - 3
E4 - Oxy Sensor Adaption Limit Cyl #4 - 6
BD - Pre Cat Oxy Sensor Heater Circuit #4 - 6
I replaced Oxy Sensors about 30,000 miles ago, but they were cheap ones.
I hate to say how old the plugs are; more than 30,000.
No recent maintenance regarding these issues. But E3 and E 4 have always been an issue.
197,000 Total miles
Thanks!

Mine recently had E3 and E4 codes. The DME was adding fuel due to detected unmetered air past the MAF. Sure enough, an air intake hose connection was loose. Check the intake hose for any loose connections or cracks. Repair as necessary.

I would only worry about the BD code after the first three are fixed. The cats may not be getting hot enough to heat the O2 sensors properly.

Quick search:
BD DME: Insufficient heat output, oxygen sensor before catalytic converter, bank 2

May I ask what DME is?
Is the "intake hose" you refer to the one that the MAF is connected to?
Sometimes small hose leaks are difficult to pinpoint. Are there certain air hoses that go bad earlier than others?
Thanks!

Actually you are only getting one O2 sensor code and that is the one for the heater circuit.
Cheap O2 sensors (or a lot of other things) can be a can of worms for sure though.

Thanks but can you please explain what you mean by "heater circuit"?

the sensor has a heater built in to get it functioning quicker that's what the "heater circuit" code is for.

DME = PCM, ECM or engine management computer
GMIII = body control module

Follow the air tube past the MAF sensor. Check all the clamp connections. Use a screwdriver to tighten them. Check the flex bended portion. Over time, a crack will develop there. Listen for any vacuum leaks with the engine on.

A quick DYI video check. https://www.youtube.com/watch?v=ue3N5Pg9yJw