@TechConnectify Ok, after digging a bit I see your point about underlying chemistry. Definitely lambda ratio oscillations have an impact on a catalyst converter efficiency https://www.sciencedirect.com/science/article/pii/S0926337323013000 Both amplitude and frequency matters. This also explains why the frequency is similar in all engines. And the frequency is quite low, much lower than lambda sensor and ECU would theoretically allow - this indicates that ECU deliberately acts this way. Even with two state sensor we can call this a proactive action. The downside of the two state sensor is that, the lambda ratio can drift during low or high signal state period. As for the graph, it should have a disclaimer like "for average lambda in X period of time". I've learned something new here;)