I have an 02 Lancer ES and over the past few months my car has started consuming way too much fuel! My engine light is on and the error code I keep getting is the P0421 which I know what it means already. I've changed both oxygen sensors and the catalyst that's under the car but I found out that there's another catalyst in the header. Is that true? Could this be the cause for the increase in fuel consumption? I'm also aware of the recall based on the P0421 code but unfortunately my warranty doesn't cover it. So what do you all think could be the problem? I change my oil and filters frequently and I also changed my spark plugs recently.

yes if the warmup cat is bad...it will if u keep getting 421 codes

So the warm up cat is probably what's bad because it's the only thing that I haven't changed. But is that a reason why it would consume more fuel?