I have an 03 Dodge Ram 1500 that keeps thowing P421 engine codes. This will come on and then go out. The last time it stayed on for 3 weeks and now it is out again. Any thoughts? I have about 76000 miles on it.

My code book says "Warm up catalyst efficiency bank 1 below threshold". I think it means your O2 sensors are not warming up fast enough. May have to replace O2 sensors. You should have them checked with a scan tool that can read live data.

Are the o2 sensors easy to change for a fairly handy person?

You have an oxygen sensor in front of the converter and one behind the converter. Your auto parts store sells a tool that makes it pretty easy to install. The hard part is getting it to break free of the rust. My rear one went out about 6 months ago. I think it was about $100. A little expensive if you are not sure that is your problem. They make cheaper ones that you have to splice the wires together but mine was a direct plug in.

Thank you for your help on this. I think I will get it scanned next time it comes on. Thanks again