Oh - I assume you know the funtion - that's air injection into the exhaust ports. It only occurs there when the engine is cold -- air injection occurs there when the engine is cold so that cold air injected into the cats won't interfere with quick cat light-off. Once the engine/cats warm up, the air injection diverter valves divert air/smog pump air to the cats. So while they might be necessary for a visual inspection -- any smog check run with the engine/cats fully warm will not be impacted at all if the crossover is missing or not working properly.