Subaru Forester Owners Forum banner

21 - 40 of 812 Posts

·
Premium Member
Joined
·
10,409 Posts
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.
 

·
Registered
Joined
·
377 Posts
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
 

·
Premium Member
Joined
·
3,363 Posts
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.
 

·
Registered
Joined
·
23 Posts
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.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #25
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.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #27
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.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #29
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?
 

·
The Modfather
Joined
·
8,097 Posts
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:

http://www.subaruforester.org/vbulletin/showthread.php?t=6945

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

·
Registered
Joined
·
24 Posts
Discussion Starter #31
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:

http://www.subaruforester.org/vbulletin/showthread.php?t=6945

Vac lines are discussed, I think there are some photos and stuff there too.
I'd prefer to use a vac line, unless it is possible to pour (not spray) sea foam right into the throttle body?

Found 2 vacuum lines that look like good candidates. Can't say I know what their names are, but I attached pictures.
  • First line goes from the "Air Assist Injector Solenoid" to either the throttle body or just after the throttle body (by what looks like some sort of sensor.
  • Second candidate runs from the purge control valve to the main chamber of the manifold. For some reason, I didn't get a picture of where it connects to the PCV.
Is one of these a better option than the other? Thoughts? Suggestions?
 

Attachments

·
Registered
Joined
·
24 Posts
Discussion Starter #33
Status update:
Was way too distracted by the beautiful weather this weekend to do any work on the car. Oh well.

Did the Sea-foam this afternoon. Used between a third and a half of the bottle. I was surprised at how long the smoke lasted. Luckily it's always windy up here in the copper country. Went on a spirited ride up the Keweenaw to clear the last bits of smoke out :icon_cool:. The rest of the bottle will probably go in the gas tank.

Heading downstate this weekend, hopefully the CEL stays off, but I'll get an emissions check while I'm down there and see if there are any real problems.

Edit: Used the vacuum hose in pics 1-3, as it was the most centrally located, and easiest to access the other end of the line.
 

·
Registered
Joined
·
377 Posts
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?
I will be interested to hear your results. I'm skeptical that SeaFoam can clean a catalytic converter, since the hydrocarbon is combusted mostly before entering the converter. However, it's worth a try as someone said.

My Eastern catalytic converters seems to be working OK so far, w/ no return of P0420. I have not run my scanner to check for 2nd O2 sensor response pattern.

good luck
 

·
Registered
Joined
·
9 Posts
UK '01 Manual All Weather Forester.

The annoying P0420 reappears between 70 and 200 miles. Two independent exhaust gas tests showed near-perfect lack of pollutants so I ended up looking at the oxygen sensors. There isn't much obvious information on these but I appear to have a wideband air:fuel sensor in the front, it has 5 wires and some device on the plug, another oxygen sensor? Is this correct?

The rear one seems conventional with 4 wires, (it's removal just brings up the "sensor heater missing" code).

I'd like to reflash the ECU to prevent P0420 appearing but, apart from anything else, I haven't found the ECU yet. On the RHS (driver's) there are the 2 green connectors but no sign of the unit with it's 3 connectors and mass of wiring. On the LHS there's mostly mouldings associated with the heating/air con. My wiring harness diagram is the US version.
 

·
Super Moderator
Joined
·
35,216 Posts
The front O2 sensor, also called the A/F (air/fuel) sensor is used by the ECU during closed loop operation. This sensor is critical for engine operation. As far as I know, it's not a wideband sensor, that would be expensive & not needed. The rear O2 sensor is used to monitor the efficiency of the CAT. The rear sensor on the '05+ is also used for fuel trim. If you're getting a CEL code P0420 & you're sure there is nothing wrong with your emissions (normal engine operation, exhaust pipe tip is not sooty), it could be as simple as a faulty sensor. Unlike the front sensor, these are not critical, so you could get a cheap one on the internet.

I don't think an ECU reset will clear the CEL code P0420, so you'll have to use a scan tool to reset it. :confused:

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 

·
Administrator
Joined
·
30,099 Posts
I'd like to reflash the ECU to prevent P0420 appearing but, apart from anything else, I haven't found the ECU yet. On the RHS (driver's) there are the 2 green connectors but no sign of the unit with it's 3 connectors and mass of wiring. On the LHS there's mostly mouldings associated with the heating/air con. My wiring harness diagram is the US version.
You mean reset the ECU? It won't clear the code... you'll need an OBDII reader like the ScanGauge II to read and clear codes. For future reference, if the SF is anything like the SG, the ECU should be under the passenger side footwell.
 

·
Registered
Joined
·
657 Posts
I WAS getting the PO420 code practically all winter. It'd come, then go away, then come back, then go away... From what I've read on previous threads, running with this code won't hurt the engine since the code is from the cat, so I just let it go.

Funny thing is...it started once the snow started falling (I swear it happened the first time we had an accumulation). And since the last snowfall, I haven't seen the code at all, especially since it's been warming up.

It's made me wonder for a while now if environmental conditions may assist in kicking the code in.
 
21 - 40 of 812 Posts
About this Discussion
811 Replies
215 Participants
2.5x_sleeper
Subaru Forester Owners Forum
We’re the best Subaru Forester Owners Forum to talk about the best years of the Subaru Forester, modifications and reliability history of the Subaru Forester.
Full Forum Listing
Top