Only when the cats where bad. Its a peculiar situation. The upstream O2 being so bad that they killed the cats because the engine had adjusted so far or leaded fuel was used, etc.

Then with new upstreams the "proper" AF mixture is being used. The spend cats are no longer working within spec and the downstream O2s read this and throw a code.

Or, I suppose, the front O2s are the wrong ones and causing the cats to work overtime.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car