P0420 - Weird Issue

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jjcool00

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Ok so I have had an issue with my Cats quite some time now. About 6 months I've had a rattle coming from below. The rattle has gotten significantly louder over time to the point where it is obnoxiously loud. I've had bad cats on cars before and was almost certain it was the same/similar sound. I was going to order the Magnaflow cats together and just change them both at the same time. The car has 114k on it and probably has the original cats on it. (I bought it at 52k and they haven't been touched since) So right before I order the new cats I change the oil (saturday). When the oil change is done I go for a drive and there's NO rattle. None. It sounds perfect. Perplexed I asked a couple of other members about this to see if anyone has ever encountered a situation like this before and they had not. So I keep driving it, and I got the windows tinted sunday. No issues, no rattle, no loss of power, nothing. Now since I cannot roll down the windows I've been using the A/C which I never do. The first day I use the A/C - Sunday I'm at a traffic light and the car is shaking as if it is going to stall out!!!! I'm thinking misfire but know it can't be. I just changed all 8 coils and plugs 10 months ago with visteon cops and motorcraft plugs. So I'm sitting there hoping a CEL would come on and it doesn't!

Now the Lincoln is no longer my daily driver so it sat until yesterday. I drove it about 50 miles... no issues had the A/C on no hesitation, no shaking, perfect - still no rattle. Today I get in it, start it and there's a CEL! I scan it and P0420 comes up. I've searched thoroughly and the consensus seems to be cats or O2 sensors. But none of the threads have any info about the issues I've been experiencing in particular so I figure I start a new thread and maybe obtain some guidance before I go buying the Magnaflow cats (which I'm thinking I need to do now)...

Is it possible the cat rattled to the point where something inside the honeycomb broke and now permanently clogged the cat, causing the code and hesitation with the A/C on? Or is it just a sensor?
 
Yes, it is possible. When you have an obstruction in the exhaust path due to something breaking, you can get intermittent problems like that as the force of the exhaust causes the obstruction to shift position and block off exhaust. Since you know the cat had a broken brick inside, that's not something that gets better with time. I would proceed with the cat replacement sooner rather than later.
 
Yep, I have a cat going out on my EXPY, it was rattling like crazy a few days ago, now, nothing. Im sure my melted honeycomb (brick) has just melted off to the side... marginal coils are not something to ignore (I HATE COILS!!!!!)...
 
Yeah I didn't even ignore the marginal coils - what happened was as soon as I noticed a misfire last year I was 200 miles away. Instead of paying 2789478789 in tow charges I drove it to the mechanic... when I got there he said marginal misfire in cylinder #4 and #7 so I changed all of them... but I guess 200 miles on two bad coils is enough!!!
 
jjcool00 said:
Yeah I didn't even ignore the marginal coils - what happened was as soon as I noticed a misfire last year I was 200 miles away. Instead of paying 2789478789 in tow charges I drove it to the mechanic... when I got there he said marginal misfire in cylinder #4 and #7 so I changed all of them... but I guess 200 miles on two bad coils is enough!!!
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They probably didn't both become marginal at the same time. You probably didn't notice the first one for some unknown number of miles. Alternately, 200 miles probably is all that it takes if the misfires were pretty bad.
 
Yep, I'm in a similar issue, Cat was done in by the previous owner so I benched it to get the parts I needed to fix. Did the coils and didn't do the cat, lost a coil a few days ago to misfire. So I'd say take it i as a top down issue. Look at the Injectors, The Throttle Body, the IAC and the CV Hose as well as getting the Cat replaced and probably new O2 sensors.
 
Yeah, good point Joe. If I recall correctly, the misfires weren't bad. The only reason I even noticed them is because I was stuck in traffic in 100 degree heat and the RPM was slightly jumping up.. maybe 100-200 RPM then going back down 100-200 RPM. I figured that wasn't normal, did some searching here - noticed coils were the issue - took it to the mechanic and had all 8 replaced.

Here's what I'm afraid of - that the A/C stutter was caused by another marginal coil and that I'm going to replace these cats only for them to get ruined again. But it seems as if I'm just being paranoid - my new coils and plugs probably have about 15,000 miles on them and if the bad cat can cause these issues maybe just replacing it will be the best solution. I'm going to have the VCG replaced on Sunday and order the cats today.
 
jjcool00 said:
...Here's what I'm afraid of - that the A/C stutter was caused by another marginal coil and that I'm going to replace these cats only for them to get ruined again. ...
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Were the gaps of the spark plugs verified to be 1mm. What coils and what spark plugs (brands) were used?
 
Visteon coils and Motorcraft plugs. The mechanic said he gapped them although I did not watch him do it.
 
jjcool00 said:
Visteon coils and Motorcraft plugs. The mechanic said he gapped them although I did not watch him do it.
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You should be safe for a while then (except for random chance failure).
 
joegr said:
You should be safe for a while then (except for random chance failure).
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Would you agree with the consensus above that the stuttering was caused by the failing/clogged cat? I'm going to order cats now.. hopefully they come in by next week.

I appreciate all the feedback as always fellas.
 
jjcool00 said:
Would you agree with the consensus above that the stuttering was caused by the failing/clogged cat? I'm going to order cats now.. hopefully they come in by next week.

I appreciate all the feedback as always fellas.
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Yes, I do agree that it is the likely cause.
 
If you are really concerned about coils killing the new cats, you could be proactive with it and replace the coils and plugs at the same time. This is the same sort of thing I normally do with an alternator or battery failure, replace both at the same time as one will invariably kill the other. You can do coils and plugs, then drive straight to the exhaust shop for the cats.
 
That's true... even though my coils/plugs are less than a year old. Where are you guys getting COPS from? I see the dealer has them but 80$ each :(
 
prophaci said:
I'd go with the Eastern Cats. They have a direct fit for half what you are getting your magnaflows for just about anywhere you look.
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Ebay has both magnaflows together for 450$... how much are Easterns? Are they really around $225 for both?
 
Someone else can tap in with experience, but from reading around here the LS doesn't like the Magnaflows.

Sorry no first hand..just second hand experience.
 
purevilpleasure said:
Someone else can tap in with experience, but from reading around here the LS doesn't like the Magnaflows.

Sorry no first hand..just second hand experience.
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Thanks for the input, I've just read about 20+ threads with the words magnaflow catalytic converter in them and haven't seen anything bad... So if anyone has had a bad experience with the magnaflow cats please let me know... In the meantime I'm gonna search around for a shop to place the ones on that are coming... The LS has had a WORLD of issues lately so this is just one of the many things I'm going to be doing this week.
 
I have one on my Silverado. 3 inch pipes off the headers, dual 3 inch into single 3 inch line, single Magnaflow car, splits into duals after the Walker Quiet Flow muffler. Works OK. No idea if it sets any codes because I have the rear O2s turned off. So far as the LS, it still has the original factory exhaust.
 
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