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Discussion Starter #1
Forester 2001 L - Stock
101k Miles
P0420 - Catalytic Converter Efficiency Below Threshold

The forester has been throwing this code for a few months now, typically shows up on long highway runs (but to be fair, that is the vast majority of my driving). I bought a scanner from scantool.net, which I am quite impressed with for $60 new. This is the only code that's been thrown, so I am assuming it's the cat, I just want to gather what insight I can before going through with the repair.

I have seen some posts saying it could be the rear O2 sensor failing. How likely is this, given it has not thrown its own code? I have called around the area, but nobody here can do an exhaust check to find out for sure. I've also heard that an emissions check might not give any useful information, since the efficiency code is thrown well before the problem gets out of hand.

Assuming it is the cat, is there a good way to determine if it is just the front or rear unit, and replace individually? Also, can anyone offer any insight on the best replacement unit? I saw TexasForester's excellent writeup on the Eastern #40237 and his issues with that unit. Does anyone have any experience with other possibilities?

Any help is appreciated... Thanks!
 

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My own experiences with this code across many different vehicles ( not only Subarus ):

1. Look for exhaust leaks anywhere from the engine to the rear oxygen sensor. Many times I have found this to be the biggest cause.

2. Failed or failing Catalytic converter. Owners usually don't want to hear this, but unless there is a leak, or slow responding oxygen sensor ( front or rear ), the code is correct when it is set. If you reset this code without fixing anything, it will usually come back in a day...2.....week....next 2 weeks....The converter should be replaced.

3. Cheaper than the converter but usually not the cause, replacing the front, rear or both oxygen sensors. Without a code pointing directly at the sensor, or charting the sensor to prove it has a slow response, this is cheaper than replacing the converter, but in most of my experiences, it just puts off replacing the converter.

I have seen converters last years and 150K + miles. But it seems the ones that were part of the 99 and newer vehicles are of a different substrate and don't seem to last as long. Somewhere in this time, car manufacturers had to lower emissions again and introduced a slightly different converter and oxygen sensors. Both don't appear to be a long lasting as in the past.


My own experiences....and $.02. Take it for what it's worth. You are welcome to input with your own[/I]
 

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Discussion Starter #3
Thanks for the insight.

My ODBII scanner looks like it should be able to chart the sensor voltage. What should I chart in addition to this to determine if the sensor has a slow response? In addition, what should I be looking for?

Another concern of mine that I should have added in the original post: given the age of the cat and the lack of other thrown codes, should I be concerned about diagnosing the cause of the failed cat, or is it likely that it failed due to age?
 

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As I said. Some I've seen fail early, some late. If you can chart the sensors, start with the front. It should stay pretty steady as you drive on a highway. If it shows a slow response when you give it gas or let off, then it is 'suspect'. Maybe not the cause of a P0420, but a good suspect non the less. If the front plots slowly and all of a sudden the CK Eng light appears, it's a good suspect. The rear can plot slow if it's defective OR the Cat Con is defective. That's a tougher call as to which is causing it to plot slowly.
 

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I monitored the performance of my 99 Forester for quite a few months before replacing the converter. I checked for exhaust leaks, attempted to "rattle" the converter, etc. My forward O2 sensor performed per guidelines w/ several low/hi voltage changes per second, depending on engine speed. My rear O2 sensor would occasionally show a brief constant voltage, but more oftern changed voltage nearly as frequently as the forward converter. What puzzled me was that I could reset the ECU and it would sometimes go for 3-4 months before the CEL would come on again w/ P0420.

Excellent link from Ferret. Read that for sure.

I would take my time and make sure you have no ignition, fuel, or air leaks before deciding. I would look carefully at Eastern, DEC, and Smith Bros (used cats) and decide what works for you.

I would be interested in your experience.

good luck
 

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Discussion Starter #6
Regarding the CEL timing: it comes back on within a few days of a reset if driven enough, but most of my driving up here is for groceries or to head to the beach, since I ride my bike to/from campus when it's nice enough. If I'm on a longer drive (to/from downstate), it comes back on within 150 miles.

Texas: regarding your experience with the Eastern cat... have you worked out the resonance problems with your heat shields? If so, please share your fix.

Thank you both for your help... now I have a better idea as to what to look for. Hopefully I will get a chance to take a look this week, if not then this weekend for sure. I will post the results when I do.
 

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A dead rear O2 sensor can definitely trigger a P0420, as that was the case with my Forester last summer.

I went to replace it and found that one of the wires had been burned through somehow.
 

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I've be battling this code too (P0420) since the beginning of this year (car currently has about 140k miles). First there were two codes (forgot the other one, it was related to the rear sensor), so I replaced the rear sensor, but P0420 is still there once in a while. I kept on resetting it and it behaves exactly like others have described: it comes back, sometimes weeks, sometimes months. It looks like the cat converter is at fault in my case.

I'll try the $5 fix (mechanical fix) first.

BTW, why do cat converters go bad? I thought the whole point of catalyst is to aid in the chemical process without being consumed by the process. Is it because the precious metal being covered in gunk (reduced efficiency?), or is it truly gone?
 

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Discussion Starter #10
While this will probably stop the P0420 code and CEL, this isn't fixing anything. From what I gather, this only takes the sensor out of the exhaust stream, rendering it useless (can't be used to check for other problems, like if your cat REALLY goes bad). This is OK if you don't have any real emissions problems, otherwise I don't recommend it.

Cats can go bad from gunk or carbon fouling (reduces surface area), from unburned fuel entering the cat (melts the catalyst and SEVERELY reduces surface area), or the cat can fracture (blockage). I suppose the catalyst could flake off the substrate, reducing the amount of catalyst in the system, but I don't know if this happens.
 

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SilentHorn, I found the resonance problem to be associated not w/ the new catalytic converter but w/ the rear exhaust pipe and muffler. Probably due to change of vibration from the lighter replacement converter. A squeaking sound eminates from one of the hangers. It is not bad and only occurs after a cold start.

Why does the Subaru converter fail? I thought about that when I removed my converter and found it to be totally intact (from end inspection only) w/ no signs of overheating or damage. I have to conclude it is carbon fouling. Likely source of carbon is the PCV system recirculating oil vapor back into the the engine. Some cars use oil scavengers to condense and recover the oil vapor.

I wonder if perhaps Smith Bros, who refurbishes converters, hasn't discovered a method of cleaning the carbon. For instance, a solvent to dissolve the carbon deposits might work very well. The Berryman Chem-Dip Carbuerator and Parts Cleaner is a good possibility. This stuff literally dissolves varnish before your eyes. It will also dissolve your hands unless you wear gloves! A gallon is fairly inexpensive, probably $20 or so. Stop up one end of your converter and fill the converter w/ Chem-Dip. Let it sit overnight, flush w/ clean water, and dry. This just might work.

Regards
 

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Discussion Starter #13
Is chem-dip safe on the cat?

A quick google search turned up cataclean... a fuel additive designed to remove carbon fouling, and looks like its sold only in the UK. It doesn't get rave reviews either. Anybody have any experience with this? Sounds like snake oil, but might be worth a shot if the fouling is minimal...
 

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According to the wiki article, the most likely cause is ZDDP, which is used extensively in all previous generations of engine oils. The current oil spec (SM) has that dramatically reduced to prolong the like of cats. Carbon fouling is unlikely since a spririted run should burn it off.
 

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While this will probably stop the P0420 code and CEL, this isn't fixing anything. From what I gather, this only takes the sensor out of the exhaust stream, rendering it useless (can't be used to check for other problems, like if your cat REALLY goes bad). This is OK if you don't have any real emissions problems, otherwise I don't recommend it.
Why not? In my experience, a P0420 is more a code to prove a bad rear O2 sensor than an actual innefficient cat.

How do I know? I threw a P0420 constantly in my Forester. I cleared the code maybe once a week. I finally got tired of seeing the CEL and looking like a NOOB on the street, so I hooked up a 5-gas analyizer. Emissions were clean as a whistle (WELL below standards).

Did the mechanical fix, not a code since, still passes Emissions with flying colors.

The mechanical fix simply slows the sample rate of the rear O2 sensor so that it's plot doesn't mirror the front O2 sensor.

I have never seen a "real" problem that a P0420 was the first code to let you know. You aren't getting rid of a code that could potentially save your engine. If the cat really goes bad, you'd smell it, and notice the fuel efficiency difference.

Get the car emissions inspected. If it looks fine, do the mechanical fix and be done with it. The rear O2 sensor's job is only to tell you if the cat is bad, and it does a pretty poor job of that. It doesn't have any effect on fueling or anything of the sort.
 

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Discussion Starter #17
I'm good friends with a higher-up engineer at Ford, he told me this with regard to Ford vehicles:

I would expect that this is true of the subie, given the environmentalist reputation of the company.

I'm not arguing that it can't be a failing O2 sensor. I'm not ruling anything out, because I don't want to preclude any options before I've had the chance to inspect the potential causes on my own vehicle (or do the same for anyone else). I just fear that the "mechanical fix" might hinder future diagnostics, and that if the cause is truly believed to be the rear O2 sensor, to simply replace it.

If it's possible that the mechanical fix can still set off the CEL if the cat degrades further, then I take back all I said about the solution.
 

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So the Ford Employee admits that the P0420 is thrown WAY before the cat is actually bad? Well that seems like a bad idea from just about every angle.

I'm not sure I know what future diagnostics would the mechanical fix prevent. None that I can see. If you suspect a potentially faulty catalytic converter, the first and foremost thing you should do is have the emissions checked. A 5-gas analyizer is pretty common for some shops, and it's similar to what state-emissions inspection stations use.

I would NEVER trust a P0420 to diagnose a bad cat, but that's my personal opinion. Ford even says that you cannot rely on the rear O2 sensor to tell you of a bad cat (since it will throw a code before anything is actually wrong with the cat).

On whether or not the mech. fix will still allow a P0420 to throw if the cat degrades further, I'd think it would eventually throw the code. The mech fix works about 65% of the time for eliminating the P0420 code on catless cars. You could likely increase the sample of air that the O2 sensor monitors, by enlarging the hole in the mech. fix, but I've never cared nor tried.
 

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If it's possible that the mechanical fix can still set off the CEL if the cat degrades further, then I take back all I said about the solution.
I have the mechanical fix on my FXT, and it still sets off the code once in a while. Not quite as frequently as it used to, but every few weeks or sometimes months. It invariably happens when the car is driven with an extremely light foot or when coasting for an extended period of time. I guess this is because there isn't enough pressure to evacuate the gases from the exhaust in a quick manner, so they accumulate and set off the code.

I fully agree with BAC5.2 though... the P0420 is nothing more than an annoyance. I had my car hooked up to an emissions sniffer at my local smog station once, simply because we were curious if it would still pass smog without the downpipe cat. It passed with flying colors, so I know for sure that it isn't my cat going bad (I have a hi-flow cat in the mid-pipe still).

I'd do the mechanical fix and be done with it. You can throw more money at it and replace the sensor and/or cat if it eases your mind, but it's really not necessary.
 

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Discussion Starter #20
BAC5.2 and pleiad7, thank you both for this information. It helps to see more experience with the subaru monitoring system.

Unfortunately, there are no stations in the Houghton area that can check my exhaust :icon_frown:. I even tried the Subaru dealership in Marquette (100mi away), to no avail. This is when I contacted my friend at Ford, see if he knew any place in the Detroit area to get this done. This is when he told me what he knew about Ford diagnostics.

In defense of throwing the P0420 early, I can see the justification... alert the driver and the service guy to keep an eye on a developing problem. However, it can be rather stressful and frustrating. I don't see any point in arguing for/against Ford's practices. What I really want to do is find out most affordable and environmentally-friendly way to solve my problem. I can't afford to throw money at it just to ease my mind (this is why I'm doing the work myself), but I'm hesitant to pursue a cheap fix if could result in more environmental damage.

I am now starting to wonder if the university has an exhaust analyzer... I know of a few student groups involved in cleaner automotive applications. Being a student here, its probably much cheaper than getting it done at a service station.

I've read that the O2 sensor works by comparing oxygen levels in the exhaust to oxygen levels outside of the exhaust. High voltage corresponds to low O2 levels in the exhaust, and low voltage corresponds to high O2 levels in the exhaust. The ECU interprets high O2 content (low voltage) after the cat as low cat efficiency. Fine, so how would the O2 sensor fail?
  • Higher sensitivity to exhaust O2 doesn't sound right.
  • Lower sensitivity to ambient O2 sounds more reasonable. Is it possible to simply remove and clean the rear sensor?
  • Internal breakdown causing a lower internal resistance somewhere could reduce the output voltage, but it seems that this would cause a complete sensor failure (no output voltage), triggering its own fault code.
I can't find any good information on the chemistry behind these sensors or how they are built, so this is just an educated guess. Can anyone offer enlightenment on the subject?

I hope noone is annoyed with all the questions. I'm in engineering... I like knowing what all my options are, and how everything works. Again, I want to thank everyone who's helping me out with this, I'm really glad I found this forum!
 
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