2000 Subaru Forester - CEL P0133, Oxygen sensor

Maintenance/Repairs
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21

Okay - here’s another update.

I took my Forester to another shop to have them attempt to diagnose the problem.

They told me that they can’t do anything because the muffler shop installed aftermarket, generic catalytic converters. With the, the placement of the O2 sensors is different than the typical Subaru converter system.

The first image is the system they say I need.

The second is what I have.

The placement of the front O2 sensor on what I have is a bit more downstream on that front converter than what is typical. Would this cause the P0133 code, delayed response?

The shop is suggesting that I need new, new converters, and a brand new exhaust system.

22

I dont know how likely this is, but since the cats were replaced i wonder if the front/rear 02 wires might have been reversed.

From the FSM for my 02 Sonata: PO133 possible causes:

  1. Front and rear Ho2s connections reversed.

  2. Faulty fuel delivery system

  3. Leak in intake or exhaust system.

  4. Faulty MAFs ground circuit.

  5. Faulty h02S.

Note: If any misfire, purge soenoid valve,MAFS or HO2Sheater codes are present do all repairs associated with those codes
before proceding with this trouble area.

23

Switching the wires for front and rear o2 sensors would cause a P0420 cat efficiency code with the slow activity code. The front sensor needs to be scoped. The engine needs to be forced from rich to lean and lean to rich by inducing a vacuum leak (the brake booster hose works good for this) then plugging the leak with propane then let it leak again. Do this back and forth a few times. Pause the scope and measure the rise and fall times of the o2 sensor. If it takes longer than 100ms either up or down, replace the o2 sensor. The fact that the o2 sensor is a little down stream from stock should not affect this measurement. Find a shop that can do this measurement.

24

The first thing I noticed in your original post was that you replaced the “front” oxygen sensor multiple times. It is not the front sensor that monitors the catalytic converter. It is the sensor AFTER the converter, commonly called the “downstream” oxygen sensor. Replacing the fromt sensor will hav no effect on a catalytic converter code.

Secondly, and I’m truely sorrry to say this, the shop that reconfigured your exhaust system to accept a generic catalitic converter did not do th ejob properly. Yes, that CAN leave you with a chronic CEL light. And, as you’ve discovered, other shops recognize that the first step to solve the problem is to reconfigure the exhaust back to original.

I believe you’re justified in returning to the mom & pop shop and having them return the system to the way it should be…and deduct from the cost what they charged you for the butchered syste, UNLESS, of course, you insisted against their advice that they do this to attempt to save money. Yes, that does happen. If you did so, they’d be justified in charging you full boat.

I wish I had better news. But that’s the truth as I see it.

25

the same mountainbike

The rear oxygen sensor monitors what the cat is doing and sets the check engine light, but has little or nothing to do with fuel trim (depending on vehicle). The front oxygen sensor has everything to do wih fuel trim and affects how the cat works. While an aftermarket cat may never be as good as a factory one, if you are not in full fuel control, the cat will not work correctly. If full fuel control is restored, even the substandard modified setup should still work, admittedly, not as good as factory.

The fact that the code is coming back makes it evident that the rear o2 sensor is working good enough. If it wasn’t, it would not be switching. No cross counts, no cat efficiency code. The cross counts on the rear o2 sensor is what sets the code.

In my shop, we have fixed many P0420 and related codes by replacing a lazy front o2 sensor, a choked up MAF sensor, a skewed MAP sensor, or repairing an intake or exhaust leak. We get the car back into fuel control then give the cat a chance to work. As a result, the shops around us replace about 5 cats a week to our one every month or two. I am in California where we have to have a smog check every two years. We don’t base our results just on the MIL not coming back on, but on the cars passing smog when we are done. I am amazed at how many of these cats wake up and work again.

A cat should last the life of the car. If it is not working, you should ask yourself “Why?”. If the why is not addressed, the new cat will fail too.

26

I have no idea how we came back around to the converter. The error code reported was P0133.

Though on that note, as much as a P0420 isn’t as simple as a report from a downstream O2 sensor, I’m not sure I’d go running around giving lectures on the subject Prof Otto. This statement: “The fact that the code is coming back makes it evident that the rear o2 sensor is working good enough.” doesn’t work at all. You also seem to operate under a notion that the converter will do something different when the car is in open vs. closed loop. The converter doesn’t “do” squat. Its completely passive - a honeycomb of metals that just sits there as a honey comb of metals. You’re just talking about other problems that throw off the AF ratios and thus mess up the contents of the exhaust. That’s obviously relevant to the converter but the converter doesn’t behave differently when the car is in closed vs open loop or when the AF ratio is at 14.7:1 or something else.

27

My apologies cigroller. I got my posts crossed and forgot we were dealing with a P0133 code not a P0420 code. But I beg to differ about what a cat does. A honeycomb of metals that is passive is useless and would have no effect on emissions. It would not generate heat. A functioning cat does affect emissions and does generate heat. It lights off unburned HC and converts CO and NOX. I have seen completely non-functioning cats (a passive honeycomb of metals) start working again with a correction in fuel control. The proof is in the pudding.

A question for the OP:

How far down stream is the front o2 from the stock location?

I am not sure if it would really make a difference. A lot of Asian cars have the front o2 sensor less than a foot from the head, yet a lot of BMWs and other cars have the front o2 several feet from the head. And a rear o2 pattern will look almost exactly like the front if the cat is not working. So, as long as the front o2 is getting hot enough (from exhaust temp or it’s own heater) the o2 should function and switch fast enough. Just curious, has anybody checked the o2 grounds and heater current yet as mentioned by others? And has anybody checked exhaust temp at the o2 sensor? Just a stray thought that ran through my head. With a heater circuit it should not matter.

28

The cat doesn’t generate heat. It has no way to do that. It gets heated up when the engine runs. And that’s what eventually makes it work as a catalyst - just heat and that’s all. The AF ratio matters only in terms of what the contents of the exhaust are - e.g. too rich and you can melt down the converter. Too lean and you might not have the right chemical mix for the catalyst to catalyze. But the converter itself doesn’t “do” anything different. It doesn’t “turn on” or “turn off” or generate its own heat or anything.

So I’ll change my language a little to be more clear. The guts of the converter is a passive bunch of metal coated ceramic. It just sits in there with exhaust flowing through it. When it is cold (roughly under about 4-500F) the guts are both passive and inert. The conditions are not present for the metals coating the ceramic to act as a catalyst. Once the exhaust heats the converter sufficiently (often 4-600F range), the metals in the converter go from being inert to having a catalyzing effect. But the converter itself and its materials still just sit there - passively - they are neither mechanically nor electrically active. They are now having a chemical effect - so you could call it chemically active. But the only key to whether or not it can be chemically active - to have the potential to catalyze - is heat which is generated by the engine.

So if you change the AF ratio it doesn’t somehow change the “activity” of the cat. It changes the contents flowing through it, in terms of heat and chemicals. This can enhance or degrade the conditions required for effective catalysis - but that’s it.

29

I tip my hat to Cig. He’s 100% correct and provides an extremely articulate explanation to boot.

The platinum-palladium alloy coating the ceramic hineycomb inside the cat converter does one thing only: when heated, as Cig said, it seperates the oxygen atoms from the nitrogen atoms in the exhaust stream, the NOx having been created by heat and pressure during the combustion process. The freed oxygen atoms then become available for
(1) the carbon monoxide (CO) molecules to grab an extra oxygen atom and become CO2
(2) unburned hydrocarbon molecules (HC) to split and grab oxygen atoms (the so-called “second burn”) and become CO2
(3) excess now-freed oxygen atoms to flow out as harmless oxygen

Thus, the exhaust out of the cat converter contains higher levels of unbonded oxygen atoms (actually, diatomic oxygen molecules are also present).

The downstream sensor simply measures the amount of oxygen in the converter’s output, just as the upstream sensor does, and the ECU compares it with the oxygen levels being detected in the upstream sensor. If the leves aren’t sufficiently different, it trips a code.

As Cig clearly said, the only source of heat that the converter has is the exhaust stream. It simply sits there and gets heated up. Platinum-palladium’s catalytic action is directly related to its temperature and its surface area, so it needs to get hot to work. And a catalyst by definition effects change without itself changing. The platinum-palladium never changes. It’s effectivity does, however, get degraded if it begins to get coated with exhaust byproducts. It needs to directly contact the NOx molecules to work, and contamination reduces the direct contact.

The upstream sensor does, in fact, affect trim levels. But the only affect it has on the catalytic coverter is if it’s providing an inappropriate signal and causing excess fuel to be delivered (cauing too long a pulsewidth in the injectors), which can manifest itself as carbon residue on the platinum-palladium and adversely affect it’s ability to seperate the NOx molecules into nitrogen and oxygen.

The MAP sensor, the MAF sensor, and leaks can in fact affect proper fuel trim levels and can, by causing desposition on the catalyst, cause the catalytic converter to be unable to its job, but it’s an indirect cause & effect rather than a direct one. The sensor has no direct effect on converter performance, it has no direct control over it.

30

the same mountainbike

Bravo! That was so eloquently stated. I am used to having to explain things more simply. Most of the customers I deal with would not understand a rocket science answer. I did not intend to come across as thinking that there was a direct connection to the cat.

You and cigroller now both stated in different ways that changing tha A/F ratio can have an affect on the ability of the cat to catalyze.

“So if you change the AF ratio it doesn’t somehow change the “activity” of the cat. It changes the contents flowing through it, in terms of heat and chemicals. This can enhance or degrade the conditions required for effective catalysis - but that’s it.”

“by causing desposition on the catalyst, cause the catalytic converter to be unable to its job”

This is what I was trying to say.

And cigroller, yes the heat comes from the exhaust but once the cat “lights off” or begins to catalyze, the catalytic affect does generate heat, generally in the 150 to 190 degree range. Volkswagen even has a spec of 150 degree C difference. Measuring tailpipe temps pre and post cat is a valid test of the cat, but not complete. You had stated “too rich and you can melt down the converter. Too lean and you might not have the right chemical mix for the catalyst to catalyze. But the converter itself doesn’t “do” anything different. It doesn’t “turn on” or “turn off” or generate its own heat or anything”. Then how do you explain the temp difference. If it did not generate it own heat through the catalytic process, where does the heat come from. I mean no disrespect, but in one breath you say that A/F ratio has no affect and in the next you say that it does, which is what I have been saying all along.

31

“It doesn’t “turn on” or “turn off” or generate its own heat or anything”. Then how do you explain the temp difference. If it did not generate it own heat through the catalytic process, where does the heat come from. I mean no disrespect, but in one breath you say that A/F ratio has no affect and in the next you say that it does, which is what I have been saying all along. "

Yes - the chemical processes generate some additional heat.

I think that what I’ve been saying is quite clear, and quite accurate - about the cat and AF ratios, etc. and at this point you’re coming down to argumentative.

Lets just say that your initial post about it in this thread was a) insulting to my e-friend moutainbike for no good reason; b) contained enough that was completely misleading or inaccurate in its implications that I decided not to leave it out there that way.

But how about we just say that the entire thing boils down to semantics - you wrote in the common-sense sort of way that you’d talk to your customers, and its true that you can’t always make things both precise and understandable at the same time. We all then went on to splitting hairs about what it means to say that something is actively “doing” some kind of work.

32

The temp differences are simple to explain: the ceramic honeycomb.

Ceramics are strange things. They’re terrible at transmitting heat energy. Thus, they make wonderful heat insulators. But that same characteristic means that they also are terrible at dissipating heat energy. They continue to heat up, without releasing the energy into the passing exhaust.

There’s a NOVA special on PBS a part of which discusses the protective tiles on the nose of the Shuttle. One of the scientists heats a block of the protective ceramic to extreme temperature, temps in the 4-figure range. He then removes the ceramic block from the oven, sets it aside for a moment, and picks it up by the corners with his bear hands while it’s still glowing red. Because the ceramic is so poor at dissipating the heat energy, his hands aren’t burned.

33

Glad we could come to some sort of agreement. I had not meant to be insulting and I admit that I have spent too much time “watering down” explanations to a way customers can sort of understand. Then when they asked me why their (fill in the auto part) failed, I like to tell them it’s how they drive, kidding of course.

I had not seen the Nova episode with the glowing ceramic block, pretty interesting. I have some stuff called Hot Stop. It’s a semi liquid heat sink. Put a 1/4" bead around a wrench and heat one end with a Victor Wrench to over 1400 degrees, you can hold the other end with your bare hand, no temp change.

34

Please don’t feel it necessary to apologize. IMHO you posted an excellent post that spoke directly to the issue at hand. Besides, I’m basically a simple guy at heart. Through these inrechanges we all learn, myself being no exception.

Hot Stop? I’ll have to check that out. All this time I’ve been using a wet rag.

35

Freaking new age thermodynamics. I should use this to heat my house. I put in a gas stream at 600 degrees and heat up a PASSIVE NON REACTIVE INSULATING SURFACE(as claimed) TO 180DEGREES hotter than the incoming gas stream.
Next thing you two will have solved the time machine and warp drive problem. All by just reinventing thermodynamics. Really an astonishing finding. I am surprized the inventor of the catalytic converter missed this amazing feat given all of the sciency stuff they did in engineering the first one.

36

That isn’t exactly correct. In reality, the ceramic doesn’t get any hotter than it’s energy sources. Its heat energy sources are (1) the exhaust, typically coming out the port at about 800F, and (2) the chemical reactions taking place within the honeycomb, as Prof Otto pointed out. It continues to accumulate the heat energy from these sources and doesn’t release it. Cylinder temperatures can reach 2500F. Exhaust comes out the port at roughly 800F or more, the balance having been dissipated into the water jacket and engine parts. The chemistry of tearing apart of the NOx molecules also releases heat energy.

Remember that heat in the combustion process comes primarily from the tearing of the hydrogen atoms from the carbon atoms. Just as heat energy is released from this chemical process, the tearing of the nitrogen from the oxygen in the catalytic converter also releases heat energy.That heat energy is absorbed by the ceramic substrate under the platinum-palladium.

How do I know all this? Because I come from the future. Via my time machine.

37

Ok Heat energy absorbed goes into heating the cat converter and this rise in temp results in a larger surface area releaseing heat energy. The cat is much larger than the pipe. Its temp as a surface measure is not hugely greater but a larger number of watts is being dissipated due to exothermic reduction of pollutants. Your simple presentation misses the actual math. and still does you really do not get the chemistry in terms of thermodynamics. This is not a chemistry course I could teach this but… Mountainbike you clearly should not go into this tech without reading the foundational issues in cat technology. I did and your statements missed the mark by not getting the transfer of heat and the chemistry of the reaction. BTW this research took about 15 minutes with the right questions, on google.