P0420 - Catalytic Converter Efficiency Below Threshold - Page 2 - Subaru Forester Owners Forum
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post #16 of 602 (permalink) Old 07-18-2007, 07:52 AM
 
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Originally Posted by silenthorn View Post
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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post #17 of 602 (permalink) Old 07-18-2007, 08:24 AM Thread Starter
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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).
I'm good friends with a higher-up engineer at Ford, he told me this with regard to Ford vehicles:

Quote:
I can't speak for Subaru's, but Ford's with a P0420 code typically need a new cat to fix it. You won't pick it up on a shop tail pipe check unless its really bad because we set the code (by reg) when its still relatively clean via that measure.
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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post #18 of 602 (permalink) Old 07-18-2007, 09:02 AM
 
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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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post #19 of 602 (permalink) Old 07-18-2007, 09:08 AM
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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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post #20 of 602 (permalink) Old 07-18-2007, 10:33 AM Thread Starter
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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 . 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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post #21 of 602 (permalink) Old 07-18-2007, 01:12 PM
 
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The problem with having the sensor throw a code so that the service guy can "keep an eye on it" is, I dunno, annoying? Good, alert the service guy. But they can't turn the light off or change it's sensitivity. I dunno. There is probably some reason.

Here's the thing I've noticed about Subaru's. Unless something is catestrophically wrong, they aren't going to be horribly polluting cars. My 1994 Turbo Legacy passed the Maryland State Emissions test with no cat's what-so-ever. Totally catless, their sensors barely picked up trace amounts of anything remotely harmful even on a load-dyno, and that was with some 99,000 miles on it!

You will notice other symptoms of a bad catalyst far before the car begins emitting dangerous levels of anything. First and foremost, you'll probably notice a change in smell, followed closely by a change in fuel-economy. At that point, you may consider getting a new catalytic converter.

I too am an engineering student. I like to explore every possible option and do my research so I can make as few compromises as possible.

In all seriousness, the P0420 is only good for making your wallet lighter by either ~$100 for a new sensor or several times that for a new cat. I've never seen a P0420 actually point to a bad catalytic converter.

As far as how an O2 sensor works, I'm not entirely sure. There are a bunch of different types using 1, 3 or 4 wires. As far as I know, and I'm not really sure, it is essentially a miniature fuel-cell system. Exhaust gas passes through an element made of ceramic that contains platinum electrodes. From this sample is what powers the fuelcell (which is based off of Zirconium Dioxide). The voltage generated is, in fact, a comparison between outside air and exhaust gas. Low voltage means lean, high voltage means rich. BUT, here's the kicker. The ECU doesn't care what the rear O2 sensor sees or reads. All it does, is compare the numbers and their rate of change to the front O2 sensor. If the rear sensor's readings and output approach the front O2's readings and output, P0420 shows up thinking that there's not enough change from the catalytic converter.

So what might cause a failure? Contamination is a very big one. Race-gas kills O2 sensors because it coats them with lead. Same with oil residue and carbon deposits. They coat the sensor and cause problems.

But what about the front sensor? If it is coated and gross from buildups of carbon and oil, it would cause a delay in reading which could slow it's response time sufficiently enough to cause a P0420. That's not outside the realm of possibilities, is it? The ECU only monitors the rear O2 sensor for catalyst efficiency. It would never know that something was wrong with the front O2 sensor. Something that could be evidence to this would be poor gas mileage, espically in the city (as the ECU would respond too slowly to an overly rich condition). Since the ECU err's on the side of rich anyway, all the O2 sensor really does is let you know when it's WAY too rich (or if something is wrong, it'll say lean. If the O2 reads lean, it's probably too late anyway).

Here's what I'd do. Call around and find out if any shop has a Subaru Select Monitor. One of the older units with the cool graphical display. Plug it into the OBD2 port, and it will allow you to monitor O2 sensor output graphically. You should be able to note a significant lag in front O2 sensor performance, and that could help you narrow it down.

If that's too time consuming, or no one has one of the already rare Graphical Select-Monitor's, simply pull out your front O2 sensor and inspect it.

My Legacy only had 1 O2 sensor. Since I had no cat's, it was effectively getting abused as if it was the first Oxygen sensor. Like I said, when I pulled it out, it was REALLY gunky after 115000 miles. I'm sure there is some spray that is safe for Oxygen Sensors. I know there is some stuff called "Electrical connector cleaner" which is just a diluted spray of isopropyl alcohol (I think), so that couldn't harm it very badly. I don't know why some non-chlorinated brake-cleaner wouldn't work either. There might be some residue left over, but the O2 sensor heater should be sufficient to burn that off. I think the O2 sensor heater brings the sensor up to near 275-ish degrees celsius.

Just throwing some ideas out there. I'd strongly suggest pulling that front O2 sensor and simply looking it over. I could be wrong, but if I'm right it'll save you a lot of headache, time and money. If I'm wrong, at least you ruled out one possibility.

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post #22 of 602 (permalink) Old 07-18-2007, 04:32 PM
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From Wikipedia "Silicon poisoning in automotive applications is the result of engine damage, such as a faulty cylinder head gasket or cracked casting, admitting silicate-containing coolant into the combustion chamber."

The 2.5L engine is well known for head gasket defects, and in fact I have noted a very slow but distinct loss of coolant w/ time in my Forester. I would say perhaps a couple of pints per year. I have searched for any leaks and can't find them. I installed the coolant system leak seal from Subaru. If coolant is getting into the exhaust even in small quantities, it may supply enough silicon to damage the converter in the long run.

regards

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post #23 of 602 (permalink) Old 07-18-2007, 05:06 PM
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Also bear in mind, Silicone Sealer was used for years all around car engines especially in gasket areas.

Well the fumes from that sealer will poison A/F ratio sensors ( the front Oxygen sensor ) as well as poison the Catalytic Converter.

In the mid-90's, many vehicle manufactures were not aware of this until they had early fallout of the 3 way catalytic converters. It wasn't until after 2000 I finally saw this as a warning in some Mopar FSM's.

So using that sealer anywhere on the engine including the oil pan is a no no. Since the oil fumes are fed via the PCV to the combustion chamber.

I do see it used on automatic transmission pans, but some manufacturers have even gone back to supplying a gasket and discouraging the use of RTV anywhere in the drive train area.

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post #24 of 602 (permalink) Old 07-18-2007, 06:28 PM
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Could be your Front O2 to begin with....

I had a previous CEL problem where the dealer told me the catalytic converter is bad, needs to be replaced. I asked how they came to the conclusion, I was told because of the CEL code. I told them to leave it alone and did my research. Here is what I found in general with O2 sensors and catalytic converters.

Catalytic converters are passive devices, that is all you have to remember for now. They are like your air filters. There is a Front O2 and Rear O2 sensor. The front O2 measures/reads the exhaust gases before it passes the catalytic converter. This measurement creates a reference voltage for the ECU to base if the Catalytic Converter is functioning properly from what the rear O2 sensor will measure/read after the exhaust gases have passed the catalytic converter. Remember now, catalytic converter is passive, does not do anything but let exhaust gases pass through no matter what the condition is. If you have a wrong reference in the front O2 because of bad sensor, you will be getting a CEL even if the Catalytic converter and rear O2 sensor is functioning properly because of bad reference to begin with.

Proving this is the case is a very time consuming process because the readings should be done and compared at the same point in time reference for both front and rear O2 sensors.

For practical purposes, it is simpler to just replace the front O2 sensor to be assured that you have a good reference for the ECU to compare specially when you encounter this type of intermittent failures.

Hope this info helps.
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post #25 of 602 (permalink) Old 07-18-2007, 09:30 PM Thread Starter
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Did some driving around today with the laptop and the elmscan ISO. I was hoping to log the output of both O2 readings in real time. Unfortunately, the polling rate was too slow (~1Hz) to capture the rich/lean cycles. Also, once I had everything hooked up, it turned out the software wasn't able to read the rear O2 sensor (I'll find a solution later). I took the car for a drive, and here's what I found:
  • Average reading under steady driving: ~1.5V
  • Voltage increases significantly when i pull off the gas at high speeds: ~4.1V

These voltages don't seem reasonable. I've read that the voltage should oscillate around 450mV, or is this only true of normal O2 sensors? I thought the primary output of the wideband sensor was current. Anyone know what the load impedance is, or what the voltage range should be? Is the impedance designed so that the voltage out conforms to the 0.1-0.9V standard? I'm a bit thrown off by these readings.
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post #26 of 602 (permalink) Old 07-19-2007, 08:25 AM
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what about seafoam?
Cheap, easy and works rather well (worked for a p0400 and p0420 on my Impreza)

It's $6.50 and well worth it to at least try (even if it does not work, at least you cleaned the bejeezus out of the engine)
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Cheap, easy and works rather well (worked for a p0400 and p0420 on my Impreza)

It's $6.50 and well worth it to at least try (even if it does not work, at least you cleaned the bejeezus out of the engine)
Finally an option cheap enough to try that its worth a shot before any more diagnosis. I assume this should be available at my local parts store (even in backwoods Houghton).

Do I just use it as a fuel additive, or add directly to engine? Is there a line I can use to siphon it directly into the engine? The directions on the website are a bit confusing (don't know if these are step-by-step directions, or a list of different applications)

If anyone could provide me nice detailed step-by-step instructions, I'd appreciate it. Not only how to get it into the engine, but what state the car should be in before, during, and after this process. Forgive me, I haven't done any engine work before. I trust the product and my ability to find my way around the engine, but want to make sure I don't damage anything.
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post #28 of 602 (permalink) Old 07-19-2007, 10:37 AM
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post #29 of 602 (permalink) Old 07-19-2007, 03:21 PM Thread Starter
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Thanks Peaty, excellent instructions!

Went through the process described here. Didn't use SeaFoam (couldn't spray), but a similar carb cleaner product. Didn't notice any white smoke in the exhaust. Did I not use enough of the product, or does that just mean there was no problem? Maybe the white smoke was gone before there was a chance to cut the engine?

The can of sea foam says to feed 1/3 can through the vacuum line. Should I go ahead and try this? If so, which vacuum line?
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post #30 of 602 (permalink) Old 07-19-2007, 03:46 PM
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It's really best to put it through the throttle body so you can get it as evenly as possible in each cylinder. I found that at least on my XT there wasn't one located in a perfect position. If you slog through this thread:

Subaru upper engine cleaner

Vac lines are discussed, I think there are some photos and stuff there too.

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