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Old 05-29-2009, 11:37 PM   #1
nti_hazard88
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Default Compilation of UEL Header Solutions for P0420 CEL

Intro:
Ok, after doing some research on this and asking lots of questions both on this forum and elsewhere, I think I've arrived at a *possible* (I will not be held liable if it doesn't work for you) set of solutions for the P0420 CEL that is caused by aftermarket UEL headers.


Background:
The P0420 CEL code means there is something going on with your cat system, either in the cat itself, or the O2 sensors. For most people that get it after installing a UEL header, there's probably nothing physically wrong (but still check for leaks anyway, to be sure), but what is triggering the light is that the new header does 2 things:

1) Exhaust gases now travel MUCH faster, so they have less time to heat up the cat section
2) The aftermarket UEL's, unlike the OEM, do not have additional heatshielding, so heat is lost from the header more quickly.

Because the sensors are picking up a lower temperature, they generate a false reading based on a faulty assumption that the gases are not hot enough for the catalytic converter to be working properly (although technically, lower temp gases won't be converted as well as higher temp; HS Chemistry class for you...). This in turn signals the system that the catalytic converter (I will call it "cat" for short, like most people do) is not working 'efficiently', throwing your P0420 CEL code.


Known solutions that (probably) DON'T work:
1) Simply adding a high flow cat (HFC).
--Edit--
The conclusion for the reason this doesn't work has been changed. Exhaust velocity isn't going to change, but temperature will, due to less insulation. Nevertheless, it isn't going to help.
- credits to williaty

Other Solutions: --Edit-- (these are solutions that I have heard of, but am not sure about)
1) Electrical "resistor" method - not very sure about this. Something about splicing and adding a resistor to the O2 sensor to artificially boost the voltage. Makes sense, but I don't know. Voltage = Current x Resistance (V = IR), in case anybody is wondering how more resistance = more voltage.
--Edit--
- williaty has noted this won't work (see reply below)


Solutions that have been "reported" to work: --Edit-- (whether alone, or when used in conjunction with others)
(Again, I must add, these are from people I know who own Subies. I can't guarantee them for you, but based on theory, using all of them in conjunction with each other should help more).

1) Heat-proof painting the headers. Using a REPUTABLE heatproofing paint (like VHT), apply LOTS of thin coats, until it is thick. The theory is that, more paint = more heatshielding, so less heat is lost from gases in the header to the outside, keeping exhaust gases hotter, and your O2 sensors happier. If you cure the paint properly, it becomes chemical resistant, which is very useful if you also want to do #2.
--Edit-- *note*
This ALONE, probably will not work. If you're going to do this, use it in conjunction with #2; chemical/salt resistance is a huge added plus, and there's probably at least some degree of added heat resistance.

2) Heat-wrapping the headers. You can get these wraps off eBay. Chances are, you'll need around 50' of it to do it properly. (BE VERY CAREFUL NOT TO WRAP UNEVENLY). While exhaust headers are (usually) made out of Stainless Steel and should be fairly resistant to chemicals (e.g. salt), if you want extra peace of mind, you should also apply #1, the heatproof paint, and properly cure it. This will counter the fact that the heatwrap absorbs and holds the chemicals when it's wet (also wash your car often to keep the concentration low). Not to mention, the paint will also help add to the heatshielding effect. Same theory as #1 - more heatshield = less heat loss = hotter gas = happier O2 sensors.

3) Spacing the REAR O2 sensor. This works off a different theory than the previous 2. By moving the O2 sensor a little more out of the way of the exhaust gases, it will detect less "imbalance" in the exhaust flow. It also has a temperature theory as well. If heat is being transferred to the header (metal is a conductor of heat), then the header will be pretty evenly hot. The O2 sensor is placed in a 'pocket' where air isn't flowing as much, meaning the air in the pocket is being held at a temperature closer to that of the metal surrounding it, which is, to say, hotter than the flowing gases. Same principle as why you have a fan on during a hot day; moving or flowing air/gases move heat away from a high concentration. No need to get into deep physics. Spacing can be accomplished by: bolt-on spacers (search forums about those), weld-on bungs at your local shop, OR, some headers provide this bung pre-welded with the header. Again, the usefulness of this method is enhanced by the other 2 previous methods used together, as keeping the heat in will keep the metal hotter, etc, etc.
-- Edit -- *Important*
williaty has noted to me that you must NOT do this on an 05+, because the ECU is programmed/setup differently such that you will have fueling issues if you move the O2 sensor from the exhaust stream.

--Edit-- *added*
4) Flashing the ECU - clears the ECU's P0420 code permanently
- credits to williaty for this one; I've seen it elsewhere, but he addressed this directly.


Final Word and "Proof"
While I am a noob on these forums and ask more questions than I answer, I am simply posting this compilation of answers I have received from other people. I myself will be (in the next few months) getting UEL's for my 2005 RS with an N1 catback running the stock cat, and I will utilize ALL of these methods together. I shall post my results when they are in.

However, I do have some proof of these "fixes" working. In my inquiring and research, I asked a friend who owns a 2004 RS, that bought the car new, and threw the entire exhaust system (header, hfc, catback) at 5000 miles, did all of the above at the time of installation (painted 8 coats of heatproof on headers, heatwrapped them, welded on an O2 bung), and has NEVER seen the CEL come on, EVER. Take into account that we live in Michigan where we see below zero Fareinheit temps, he has a 30 mile highway commute to work, AND his car is now at 123,000 miles on it (he likes roadtrips), it certainly is evidence enough for me that these theories have worked, at least for HIM, in practice. Again, I don't guarantee it for EVERYONE, and certainly do more research, but I will be doing this soon (yes, I will be doing ALL of the methods listed above), and I'll let everyone know.


--- Notes ---
Please feel free to make corrections or express any concerns about the above, either by posting or PM'ing me. Note, while I have done the research, I'm relatively a noob with Subies, so I probably won't be able to answer any QUESTIONS. I also want to note that my friend has kept his car, for the most part, STOCK (no further tuning except brakes and a lightweight pulley), so I don't know how this will work for people who do more serious tuning (cams, bolt-on turbo, etc). Anyway, thanks for reading. If you try it, I hope it works for you. And again. If it DOESN'T... I will not be held liable/responsible for it, and any damage you may cause to your car as a result of improper application. (I'm just posting a compilation of answers, and I try to explain them technically with physics and chemistry!)


--- Pending reply posts, updates, and edits/revisions ---
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Last edited by nti_hazard88; 05-30-2009 at 02:21 PM. Reason: Info edits; additions, notes
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Old 05-29-2009, 11:39 PM   #2
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wow. I haven't read it yet, but this is srs bsns.
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Old 05-29-2009, 11:41 PM   #3
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okay - so is this just a compilation of the ways you can fix the CEL caused by going catless and/or using UEL Headers?
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Old 05-29-2009, 11:43 PM   #4
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I don't know about going catless, but yes. It's more of a compilation, and I'm trying to explain the theory behind WHY they (would) work. Note, the friend I reference did ALL of the above. All 3 methods together. Welding first, then 8 coats of paint, then heatwrap.
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Old 05-29-2009, 11:56 PM   #5
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You get the same CEL code from going catless
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Old 05-29-2009, 11:59 PM   #6
nti_hazard88
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well yea, what I'm not sure about is how much of a difference going catless VS HFC is. if the exhaust flows MUCH faster in catless than HFC, it might get to a point where pretty much nothing is really going to "fix" it short of disabling the P0420 code completely, or finding some other way to cheat the sensors. That's what I meant to say. I didn't do much research on going catless, because being "environmental" and all... not a big fan of the NOx gases that are produced as a result of no catalytic converter.
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Old 05-30-2009, 12:07 AM   #7
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Yeah, nor am I. But it was a worthwhile trade-off to me. I planted a bunch of trees to compensate.
I'm serious.
Anyway, the O2 sensor "spacer" fix works to remove the CEL code on catless systems.
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Old 05-30-2009, 01:16 AM   #8
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Quote:
Originally Posted by nti_hazard88 View Post
1) Simply adding a high flow cat (HFC). You're going to make the exhaust gases travel even FASTER, and since that was the problem in the first place, what's that going to do?
Incorrect conclusion. You are having no effect on the velocity of the exhaust gasses. HFCs have less surface area and less insulation. Both lead to less conversion of emissions.

Quote:
1) Electrical "resistor" method - not very sure about this. Something about splicing and adding a resistor to the O2 sensor to artificially boost the voltage. Makes sense, but I don't know. Voltage = Current x Resistance (V = IR), in case anybody is wondering how more resistance = more voltage.
Won't work due to what the ECU is actually looking for.

Quote:
1) Heat-proof painting the headers. Using a REPUTABLE heatproofing paint (like VHT), apply LOTS of thin coats, until it is thick. The theory is that, more paint = more heatshielding, so less heat is lost from gases in the header to the outside, keeping exhaust gases hotter, and your O2 sensors happier. If you cure the paint properly, it becomes chemical resistant, which is very useful if you also want to do #2.
Probably won't work. I've had coated headers and they still shed enough heat to trigger a P0420.

Quote:
2) Heat-wrapping the headers.
This probably will work if you have the stock cat section as your heat retention should be as good or better than stock.

Quote:
Spacing the O2 sensor(s)
First of all, in any year, you MUST ONLY SPACE THE REAR SENSOR.

Second, You must not do this to an 05+ car! The ECU uses the rear O2 sensor to influence fueling. Moving the rear O2 sensor out of the exhaust stream WILL cause significant fueling errors.

The ONLY acceptible way to defeat the P0420 CEL on an 05+ is to have the ECU reflashed with the code disabled.
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Old 05-30-2009, 02:45 AM   #9
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interesting... I'll take your word for the spacing part.. I'll check again with my buddy and see if he's had any issues. but then again, he is running the 04. I'll make the appropriate edits. As for the coated headers part, I haven't heard of anyone do that ALONE and getting it to work, but doing WITH the wrap should help, and at least make it salt resistant. That's probably the primary reason for coating it. With regards to the "resistor" thing, I listed it under "other" methods which I don't really know about in detail.. Thanks for the input. Look for the edits.
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Old 05-30-2009, 02:47 AM   #10
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With an 04 or older, you can space the rear, and only the rear, away. That's why I wrote my post the way I did.
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Old 05-30-2009, 03:03 AM   #11
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LoL, thanks.. any idea how much the reflash costs?

Oh, and I'm not trying to seem smart or like I know it all. Just trying to explain certain things as best I can from my understanding, and gathering data that seems scattered, so other people who are looking around won't have as hard a time.

Thanks loads for the corrections, though =D
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Old 05-30-2009, 03:07 AM   #12
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Reflash costs whatever the tuner feels like charging you.

If I like you and I'm having a good day, I'll do it for free. If you annoy me or I'm grumpy or I want a new car part, it's $50.
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Old 05-30-2009, 03:09 AM   #13
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LoL, seriously? I'll f***ing drive down to ohio to have you do it for me if it's 50 bucks... LoL... at least IF I end up getting the CEL anyway.. I'll let you know =P
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Old 05-30-2009, 03:14 AM   #14
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Well, that was a joke. Not being a professional tuner, it's not like I have a set list of prices I advertise.


But yeah, turning off the CEL takes less than 10 minutes. If you're ever in Ohio, look me up, we'll get it fixed.
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Old 05-30-2009, 01:34 PM   #15
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hahaha... alright. Thanks. anyway.. might be a bit of a pain, but... what do you need to flash it yourself? Is there a set of tools/software I can get somewhere to handle the problem at home? And I'm not talking about a quick CEL clear. I'm looking at flashing it out like you mentioned.

--Edit--
Would this work?
http://forums.nasioc.com/forums/show....php?t=1410201
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Old 05-30-2009, 01:48 PM   #16
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RomRaider is the path to Nirvana.
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Old 05-30-2009, 02:20 PM   #17
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alright... guess I'll just have to find the right mapping and stuff to turn off/adjust the P0420.

ONE LAST QUESTION (I think)

I really, in the end, kinda want the EL headers, and maybe deal with welding the cat section later, but with UEL's... how much louder does it get INSIDE the car? I know it's going to be craploads louder when the engine is under load, but how about highway cruising? I really prefer it not to be too loud =S... I'm just curious. Actually, with EITHER EL's or UEL's... during cruising, will it be louder to the point I have to crank my music up? Because that would suck. Hahahaha.
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Old 05-30-2009, 02:23 PM   #18
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If you don't have an exhaust leak (/me looks shifty), it's no louder in the car at all. Overall loudness really depends on what you do with the muffler.
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Old 05-30-2009, 02:33 PM   #19
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Oh.. sweet! Yea, because I remember after throwing on the catback it already got a bit louder... and then it got a bit louder (when under load) after the Injen CAI... which is why I REALLY don't want it any louder =( .... I guess I'll do some more research on just how MUCH of a difference in performance the EL has over UEL. Someone mentioned you actually had a detailed thread with dyno sheets on that.. I'll see if I can find it.

--Edit--

I also want to note that I don't really care much about seriously tuning the car. I won't be upgrading the cams or anything. When it's all said and done, my final list of mods is going to be:
1) Headers (UEL/EL TBD)
2) Catback (installed)
3) CAI/SRI (installed)
4) Drilled brake rotors /w new ceramic pads (still to come)
5) Lightweight pulley

I'm not into serious tuning, because I don't intend on racing or anything. I just want to have some fun here and there. And (I KNOW I'm going to get flamed for this so apologies if it makes me seem noob) I have an auto transmission because I share the car with my 16 year old brother who doesn't drive stick. Anyway, the point is, If EL and UEL only make a really noticeable difference one more heavily tuned cars, then I might just go UEL to save myself the welding hassle, since this is pretty light tuning considering what other people do.

Last edited by nti_hazard88; 05-30-2009 at 03:22 PM.
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Old 06-01-2009, 08:59 AM   #20
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Interesting stuff you have here.

And, BTW, you might want to read the thread about the CAI/SRI issues with the '05s as well (sticky at top of this forum). Especially if you do decide to go down to Ohio, and don't want to see what a can of "whoop-ass" really looks like...

I would say there is still a difference between EL and UEL, without going too far with tuning and such, too (my opinion). But it's all up to you, anyway.
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Old 06-01-2009, 11:55 AM   #21
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hahaha, if anyone says they'd whoop my ass, I'll take their word for it. Like I said: my car isn't being tuned to the point where I'd want to race anyone. I just want to mess around. I'll have a good read on the CAI/SRI.
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Old 06-19-2009, 10:42 AM   #22
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Default Dumb question

Do cat sensors pick up temperature, pressure, or o2?
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Old 06-19-2009, 04:00 PM   #23
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None of the above. The front and rear sensors measure lambda (ratio between remaining air and fuel). The ECU expects to see a specific offset and time delay between changes in lambda on each side of the cat. If it doesn't see what it expects, it decides the cat is dead.
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Old 06-19-2009, 05:18 PM   #24
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yo nti its looks like your new to the forums and a new subaru enthusiast which is always good! this is alittle off topic but i saw your trying to put a parts list together so i thought id chime in.

-borla reps sound amazing (obx uel headers are said to have more hp but sound more raspy)
-supers587 ms3 trackpipe(rs25.com) PM him hes an aweosme guy
-stromung, apexi, and greddy make awesome catbacks if you have money to spend(if your like me you buy the cheap ebay reps for like 200=])
-rallitek sway bar kits with rear endlinks ABSOLUTELY amazing!
-perrin lw pulley, seems to help alittle idk how much alittle more throttle responce maybe
-tune from williaty!!! A+++ guy to work with and also knows his stuff, if he talks, listen!
-hybrid intake(use ebay intake from 98-01 rs only like 35 bucks... plus get some couplings for like 10 bucks and your set, theres a thread on here showin how to do it)
-hella horns, more for show and to piss ppl off but they look and sound awesome haha


hope that helps some man!

also if you decide to do headers and swaybars do it at the same time because its a pain in the ass taking the headers off to slip the sway bar in
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Old 06-21-2009, 02:20 PM   #25
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so will wrapping up my borla headers eliminate my p0420 code. It came on ever since I got my headers.
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