Wasn't sure where to post this..... had a misfire a couple months back and drove around that way for a month and 1/2 or so, finally fixed the misfire but worried I might have damaged a catalytic converter. It is not spitting out any codes but from what I've been told it wont always throw out a code. Last week I noticed a noise that sounded like a detonation problem when the engine was under a light load while using the cruise control and without using it as well, so I guess what I am asking is, will a bad cat cause a lean condition or send a false reading for an improper A/F ratio? Just for a quick background, the truck is an '05 5.4 with a tuner and already burning 93 octane, I plan on getting the exhaust completely replaced with a true duel system this week, cutting out all 4 cats and replacing them with 2 high flows, and the reason for this is because my cat back system has rusted out from the muffler back, with one of the tail pipes coming completely off from the muffler, which I am thinking was helped out by the misfire. I only drove the truck for a day since noticing the rattling noise and it has been sitting ever since until I get the exhaust replaced. Any input would be very appreciated.

 

Being yours is a 2005 with a 5.4 3v....you sure its not the injectors?

 

Oh yeah, I forgot to mention, I just replaced all the injectors, COPs, plugs, fuel and trans filters and oil.

 

As you probably already know a bad injector can lead to a fouled cat.....

Get an IR temp gun and start comparing the heat on both cats or take it to an exhaust shop and let them do it....

 

Well the misfire as it turns out was a bad coil which I finally found out when it threw a code which took awhile for it to do, and I just decided to do all the coils at once and figured I may as well do the injectors (what with the problems the '05s had with those) and plugs too, truck has 189,000 on it and it was over do for that stuff. I'm glad I did, misfire is gone, truck runs 100% better and I've gained around 2.5 mpgs. As I stated before, I'm having the exhaust completely replaced, so if the cats are bad, they're getting replaced anyway. I was just curious if a bad cat could cause an issue with A/F mixture or maybe even a detonation problem since that's what it sounds like it is doing as of last week. I know I'm probably grasping at straws here, but I figured I'd ask. I couldn't find anything when I did a search on the subject.

 

Just FYI. A missfire can and will damage a cat. With no spark raw fuel is sent into the cat which then melts the catalyst inside causing it to block exhaust flow. This will not affect a/f in the sense that the first o2 sensor controls that and the rear checks the cat. Now a bad cat will send out a p0420 or p0430 code depending on which bank the problem cat is on. Only when a cat is so clogged exhaust gas cannot leave with the cat cause a misfire. At that point you'll have a check engine light and severe powerloss and missfiring

 

When a cat goes bad, you can usually hear it rattle around in the housing, so that might be what you're hearing.

 

I had a misfire back in December... Ford said I had an engine skip. I notice a faint shake at idle. You think my driver's hi flow is bad... they thought it might be?

 

I had a slight miss for a bit. Changed the fuel filter after 60k and no more miss!

 

also your truck only has 2 cats not 4. but yes the rattling sound could be the cat

but to answer your main question a bad cat will reduce performance if its clogged or melted. and will increase your emissions

 

No that's not necessarily true... 4 cat system went up until mid 2005. He very well could have 4. They switched to two 16" cats per bank towards the end of 05

 

Claw at your furniture

 

According to the Ford Service/Workshop and EVTM Oasis. The 2005 is just two converters.

They show the system = #37173 on Ford/Oasis. Nothing else for 2005. Unless they just didn't document it.

Here's 37173 -

 

Yes, mine has 4 cats. It was built in Feb. of '05.

 

That's good to know, thanks Mitch.