p1131 Code how to repair?

Question

i have a 99 b4000, it is throwing a p1131 code. From what I read it probably means it is running lean.
I have the largest k&n intake with the largest spectre cone filter that will fit in the engine compartment.
I also have magna flow dual exhaust. Would these modifications be the reason for the lean code. The truck seems to run just fine? Also among a huge list of new parts, it has a new ecm. Also checked all vacuum lines. Thoughts?

Answer 1

P1131 Upstream heated 02 sensor not switching

This code can also be from running rich not just a lean condition. There no vacuum leaks at the air duct between the MAF to the throttle body, no cuts no rips. and the PCV hose not collapse, the engine oil is not over filled

 Here a PCV hose that collapse, a collase PCV hose like this can cause a P1131