New Plugs - New DTC !
 

A while back I had what appeared to be a stuck injector, which only acted weird for about 15 miles then went away - many DTC's were present; P0300, P0172, P0174, P2196. I changed plugs, cleaned MAF sensor and disconnected battery. Everything is running very smoothly, with the exception of P0420 - Which says something about Catalyst System Efficiency Below Threshold (bank 1). What should I look in to ? Is it possible that the injector dumping fuel ruined the converter ? Or could this just be a faulty O2 sensor ? Any ideas/Info would be appreciated - Thanks - 05 5.4 FX4, 113,000 mi.

Try switching the rear O2's and see if the code follows the sensor. If it doesn't the cat is probably toast.

Well - I cleared the code and put about 250 mi on it today (large loop through Tohono O'odham IR), Fresh tank of fuel, Multiple key cycles, no DTC's yet. If it returns I will see if the DTC moves when swapped around. I miss Carburetors and distributors - I feel lost with all this tech.