FYI:

P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)


Did you replace the O2 sensors with BMW OE sensors, or aftermarket sensors, such as Bosch?

I have not worked on BMW's very much yet, but some other makes are quite sensitive to the O2 sensor specs, e.g., Bosch O2 sensors frequently do not work on Fords, even though they are the correct Bosch recommended sensors.

Of course, it could be exactly what the code is there for -- your converters could be toast. I am not familiar with Canadian laws regarding leaded gasoline. I know some countries have allowed gasoline for marine use to continue to have some lead. Any chance you got some leaded gasoline in there that was meant for marine use?