Post head-replacement question

[Jimbo65]

So after having a cracked cylinder head replaced (cyl3 getting coolant in it), I have got a new code.
I am getting a P0420 "Catalytic System efficiency below threshold - Bank 1".
This is a brand new code, as it was not registering this even when I had the faulty head on the vehicle. I understand the Bank 1 is for CYL1 - 3, but find it very strange it comes up now, after the big fix.
Replacing the O2 sensors look like something I can tackle myself, but was hoping I could get some insight from other Forum members.
I understand that BMW recommends replacing all four at 80-100K miles (I am at 98+). I am looking to get the SERVICE ENGINE light OUT; still trying to recover from the $4000.00 head replacement and keep costs to a minimal now. Praying it is not a bad cat, is this a pre-cat, post-cat, or BOTH sensor replacement?
Any insights would be greatly appreciated!

Thanks for reading my post.

James

[Lord Vader]

What year is your model?

Odds are one of two things, either the cat is bad or the sensor is bad. On OBDII cars theirs one pre cat sensor and one post cat sensor.

[Jimbo65]

Hello, and thanks for the reply.
The car is a 2003 Z4 3.0i.
I was curious why this new code appeared AFTER the head replacement and not before? My code reader lists all codes being posted, and even during the time I had the code that was being created by the cracked head this never appeared???
I find this very confusing, as you would think this would have been an additional code posted.

[Jimbo65]

My primary goal at this point in time is just to address whatever is causing the SEL light to illuminate...it has been a curse.

[Jimbo65]

So another quick question if I may.
I was able to locate a FWD manifold/cat with both O2 sensors from a 2004 2.5 Z4. It is just one year newer than my 3.0i and am thinking this should be a direct bolt-up. Am I on the right track here?

[Lord Vader]

Some codes get stored in the computer but do not activate the SEL/CEL, but are able to be read by a code reader. When Cats go bad they usually smell pretty bad, so unless you noticed a change in your exhaust then its probably a sensor.

[Jimbo65]

Thanks for that information Lord Vader.
I was thinking about moving the sensors (upstream and downstream respectively) between the Bank 1 and Bank 2 to see of the issue follows the sensors, or remains the same.
As far as the bank 1 manifold/cat interchangeability between the 2.5 and 3.0i Z4's....do you have any insights here?
From what I understand after having to locate another 793 casting head for my Z4, the 2.5 has the 812 casting and they are virtually identical. With the main issue with the interchangeability being the camshafts needing swapped. Sound correct?

[Lord Vader]

I'm not all that familiar with the Z4, so the best way to check compatibility between parts is their part numbers. If you gotk a site like RealOEM.com you can look up the part number for both applications and if their the same number then it's the same part. Most models have a lot of cross compatibility to save costs.