Did you ever find out what was wrong with your van? I have almost the exact same description of problem with my van. I have a 2005 Dodge Grand Caravan with the P0420 code. It has 130,000 miles, and a lightly dripping pan (I just tightened those bolts and I also just changed the spark plugs and Downstream O2 sensor.) My mpg seems to be lower than it used to be but not super bad. Also, my van uses about 2 quarts of oil per oil change...4,000 miles or so.
What ended up being the problem with yours?

 

My 1998 dodge Durango 5.9L the check engine light came on. And we check the code for it and it is P0420 Catalyst Efficiency Below Threshold (bank 1).. what does that mean. and what kind of O2 sensor should I ask for? Please if you can help me out as soon as possible.. thank you for your help

thank you
Melanie

 

Catalyst efficiency is how well the catalytic convertor is working. The engine computer compares the switch rate of the upstream o2 sensor to the switch rate of the downstream o2 sensor. If the downstream starts switching too fast and matching the switch rate of the upstream sensor, it sets a code for convertor failure. Most of the time, if you have that code, you need a new convertor. I also replace the o2 sensors at that time. If you replace the downstream sensor and the upstream is old, it can also set the code because the new sensor is more sensitive than the old one. The guy earlier in this post was having issues because he was using aftermarket sensors, I recommend spending the money and using factory o2's

 

Just to let you know that I just fixed the P0420 code on my 2005 Dodge Grand Caravan this weekend. I only need to replace the downstream O2 sensor + adding the check engine light eliminator adapter (cost ~ $30 total). The youtube link is as below if you are still interested: https://www.youtube.com/watch?v=qkhDeOvvvX0&t=324s

 

This was a long time ago but I think I ended up having the cat replaced with straight pipe and called it good.