Hello all, maybe someone here can help me. I have a 2004 F-150 Lariat, 5.4l 24 valve Triton, and my check engine light is on. It's running code P0420 catalyst efficiency bank 1. I replaced both 02 sensors in bank one and it appears that both bank one and 2 catalytic converters were recently replaced. (I bought the truck used only had it for a few weeks). If its not the o2 sensors or the cats what could be causing the P0420 code and low gas mileage? (14 mpg by the way) Oh and I don't know if this is related or not but the truck occasionally blows white smoke on start up for a minute or 2. If someone could please help me I'd be very thankful!! And if not, thank you for taking the time to read this anyway!

 

14 mpg is bad?!?! Check antifreeze levels to see if it is possibly a blown head gasket and the burning antifreeze is causing the codes

 

haha, well comparing that to our other Identical, 5.4l 24 valve triton, supercrew same everything, which gets ~17-18, and I checked already for coolant in the oil and everything, and coolant level seems fine. Thank you!

 

White smoke indicates a rich mixture that's about as much as I know, what do your plugs look like?

 

White smoke is coolant. Bluish-white smoke is oil. Black smoke is a rich mixture. Generally a puff of blueish-white smoke at startup is failed valve guide seals.

Converters almost never fail without something causing it. A misfire or rich condition will destroy them.

If the improper converters were installed that may be a cause of your efficiency code.

When is the last time you changed your plugs?

 

The plugs were replaced in February, when a stuck injector got replaced

 

The most I've ever gotten is 14.7, so I don't think that's bad. Clogged cats would lower gas mileage, but they're supposed to have been replaced.