I've been battling a p0420 code for months now. I started by replacing all 4 o2, sensors, it came back on. Used fuel injector cleaner, put in a new air filter, and all new coils on bank 1, still came back on. What could be causing this code, my truck only has 120,000 miles on it so I don't think it could be the cat yet. Does anyone have any info that might help me put or what to try next? Thank you!

 

Also my truck blows a fair amount of white smoke from the tailpipe but seems to run fine.

 

P0420 is a cat inefficiency. It looks at the O2 sensor before the cat and after the cat and if it isn't with range you get the light.
So if you have replaced all 4 O2 sounds like your cats are junk.

Motorcraft O2 sensors a different than the other brands. As with other things in our trucks Motorcraft O2 are the best.
Depending on the age of your truck cats have warranties on them past the 6yr 60k that Ford gives you

 

My truck is a 2004 new body style

 

does your tail pipe white smoke smell like rotten eggs ?

 

It smells fine to me. What would rotten egg smell mean?

 

Clogged cats

 

P0420 is efficiency code and can result from a very small loss in flow in the cat. The way it is determined is the ECM monitors the up and down stream after the engine has warmed to close loop operation. If the down stream O2 does not quite down and smooth out compared to its matching up stream then you get a P0420. You either replace the cats or fool the computer with a resistor and capacitor wired in to the O2 sensor. See the Utube video: or search for $3 O2 Simulator Fix. I had the same problem in my 4.2L Freestar. I used this method and have since passed inspection and it has been in place 3 to 4 months no return.
PS: You do need to try and determine what is causing it as it can lead to more problems, however the damage to the cats is already done. Particularly if you have been driving on it for months. initially you could have tried CatacleanŽ Catalytic Converter & Fuel System Cleaner to see if it would clear it.