I just got the p0420 code last night on my 04 with the 5.4. Since ive got 4 cats and the back 2 do not have a o2 sensor behind them what would happen if i took the back 2 cats out and used one of them in place of the front cat that the code says is bad. P0420 means drivers side correct?

Im just trying to be cheap i dont feel like spending alot of money on this.

 

I would guess it is a problem with the o2 sensor and not the cat.Could be the cat but is probably the sensor.Check the connection and try switching it with the other sensor and see if it moves.How many miles are on it?I am not sure which one bank 1 is.

 

How many miles are on your truck? Your O2s are probably shot. The rear converters do not to the job of the front cats so you will get a CEL. The rears resonate sound more than they burn/filter exhaust gases.