crankcase breather and MOT emmissions

A friend of mine (ahem) -

has marginally failed on the CO emissions test.

Does the rebreathing of crankcase fumes have much effect on emissions, and could removing any of the breather hoses from the inlet manifold or air filter (and plugging the spigot?) help?

I've noticed some performance companies do actually market open breather systems to increase combustion efficiency...

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