Wife's 2013 Equinox 2.4 has had a recurring P0411 secondary air injection low flow code for about a year. Googling turns up that this is yet another delightful feature of the Ecotec 2.4, with a variety of causes. So far I've determined the electric air pump runs, because I can hear it. The condensation and ice that are known to build up in the air hoses does not seem to be the case, and the hoses aren't clogged. The check valve and carbon build up in the ports seem to be the last two possibilities, but is there an easy way to test the valve?