Late here but a spacer installed on rear 02 sensor is a common USA hack when catalyst is setting codes.
The idea is to get rear O2 out of exhaust stream to try to even out the voltage swings that signal cat isn't working. After putting a spacer in, rear O2 volt swing is greatly reduced but average voltage is usually slightly higher than I would normally expect (.7 to .8v)
When I have tried it on SI6 cars (turbo or NA) they usually start to set rear O2 codes. I am guessing these cars use rear sensor for a secondary fuel trim check and not just looking for a flat line signal around .6v to prove cat is lit and working.
