Replaced air filter - 05 trailblazer - secondary air injection sys fault?

[BreakingBad]

Replaced dirty-*** stock air filter on my 05 trailblazer with a K&N air filter.

We assumed we installed it correctly, but we're not too sure..

About 2-4 weeks after installation, a day after getting gas my check engine light came on.

Checked gas cap, went through 2 full tanks of gas already. Still on.

Got it diagnosed, "Secondary air injection system fault" was the return.

WTF could that be? Improper air filter installation? Completely seperate problem? What should i look for first after the air filter?

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[BreakingBad]

anyone?