Especial86

iTrader: (1)

If you had misfires in the passed, you very likely melted down your cats with raw gas. Which means they are restricting flow. Your problem is quite possibly the same as his...

Summers22

iTrader: (2)

After reading all this I can agree that it is most likely O2 sensors along with melted cats. Go for a drive and then see if your cats are glowing red, if so, there is your problem. I would replace the O2 sensors as well at that mileage.

jdinner

Mine were not glowing, even at night. I suspected the cats then dismissed them because my exhaust pipe had tons of steam (-22F) when cold and they didn't glow.
My other writeup:https://www.f150forum.com/f4/06-5-4-...-fires-291476/

Jr817

my symptoms are that it idles when warm at 500 rpm, doesn't really shake until i lightly tap on the gas or i go in reverse, could this be it? And if you don't mind can you post a pic of where you dropped the exhaust? Ive heard these trucks have 4 cats so i just don't wanna screw anything up

jdinner

It should have only two catalytic converters, both are about 1 foot from the engine.

Tommy J

I've looked at the cats after a long drive and they were'nt glowing but is that a sure fire test? Im sure the o2 sensors are orig. That passenger side plug looks like a real bear to get to. At the very least I could change them first

Tommy J

Is it the chicken or the egg. Are the possibly bad cats causing my rough idle and sluggishness or am I having or had at some point a misfire that caused the cats to melt. In general how do cats go bad? age or other factors along with o2 sensors. Im just ignorant when it comes to stuff I've had no experience with. You guys are way more knowlegeable than me but Im always willing to learn and by the way your set up sounds pretty good but I don't have any friends in the exhaust biz

Tommy J

Do bad cats or o2 sensors throw a code? Im guessing not. what method can I use to tell if the cats are plugged

Summers22

iTrader: (2)

I would swap out the O2 sensors that are closest to the exhaust manifold first. You are correct, the passenger side can be a challenge, but still doable without removing anything. If that doesnt cure it, then I would replace your cats.

Especial86

iTrader: (1)

For me and my 04, a misfire melted down my rear and not my front drivers side cat.. Which is weird, but it might have had to do with the fact that the front cats are angled.. So the raw unburnt fuel from a bad cop in my situation caused the fuel to blow passed the front cat and pool in the rear "level" cat.. I had some slight power issues, but the cat rattled when it was hot which was a dead giveaway for me to diagnose.

Since you have an 04 you have 4 cats.. The rears can be deleted and replaced with a piece of straight pipe.. Hopefully it's just your rears like in my case, and a delete job on the rears will fix you right up.. But without unbolting the front cats, or at least cutting the coupler between the 2 cats separating the front and rear with a sawzall, we could speculate all day.. If you run the truck with the rears severed or the fronts disconnected you will immediately notice a difference just like jdinner..

If your not getting any ball bearing sound rattling, and you already changed your plugs, and your not feeling a washboard or jerk in the transmission, you very likely have a bad cat with good plugs and cops.
If you have t changed the plugs yet, then do so first.. Use a 3/8 impact so they don't break on you with a hot motor..

If you call around your area, you can find an exhaust shop that will weld some new cats in, and replace the exhaust..