I have been getting a p420 code on my truck: Catalytic efficiency below threshold Bank 1..I will clear the code and then it will come back on the next time i start my truck. But the thing is, is that I will only get this code to come on when the weather is cold. During the summer I have no problem with it coming on at all..So i dont know what to think. The O2 sensor was changed bout a 1 1/2 yr. ago..dunno..i have a highflow cat but that shouldnt be a problem i dont think..Let me know what you think.