Secondary air injection system incorrect downstream flow. this has to do with the air pump that is driven by one of the fan belts. if the check valve is not operating or the one way valve is burned through. it will trigger this code. check the valve on each side of the exhaust if v12(one on each part of manifold in L6) and remove the hose and start the engine. if exhaust flows through replace the valve/s. this can also be triggered by excessive moisture build up inside the pump and then freezing overnight not allowing the pump to rotate(pump).
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