I have a 2004 4x4 lariat crew cab and I had the see light come on while driving home yesterday. I scanned it and got code p0420 for cat efficiency. I know my exhaust manifolds are leaking pretty good and I was going to replace them as soon as I have some extra money. I'm just curious if the leaking could cause the code or if I'm looking at cats? Thanks for your input!

 

leaking exh mani's introduce more oxygen into the exhaust before the O2 sensor, so O2 will think the engine is running lean, add more fuel, and in the end damaging the cat conv.
by letting the manifold go for too long you are probably now looking at a cat replacement as well. not to mention the fact that you are wasting fuel/losing mpg
that is worst case scenario

of course it could just be a bad sensor. either way the real fix starts with replacing the manifold(s).