Another O2 Sensor Question

[russconde]

2002 Tundra 65K miles. I've had a PO155 (Driver/Front sensor) code for many months and finally have addressed. Dealer replaced BOTH front sensors under the extended warranty. The next day my engine light comes back on and now reads PO136 and PO156, both rear O2 sensors. That seems crazy. Could it be that the rear sensors were also originally bad but only the PO155 code was being logged? Could there be some other factor that is making the ECM generate the 136/156 codes? Obviously the two rear sensors didn't ACTUALLY both fail the day after changing the front sensors. Either they were also bad all along or they are not really bad and something else is amiss.

Thanks,
Russ

[duffyatkinson]

See my post in this thread: O2 Sensor Curiosity Question

[sraney]

Duffy--since you're kind enough to read our comments and provide your learned responses, here's a follow-on to this one. As I understand it, the forward sensors are used to help calibrate the operating mixture, and the rear sensors (unheated?) are there simply to monitor the health of the catalytic converter(s). However, from this gentleman's experience (and from your previous comments about probably ending up replacing all the sensors to finally stop the codes), it appears that there is some kind of comparison testing not only side to side but from front to rear, even though the rear ones aren't directly involved in the mixture calculation. Any thoughts? Thanks.

[Highwaylizard]

I am no expert but Duffy hits the nail on the head. I can tell you from experience that O2 sensors go like women to the bathroom - in groups. I replaced all four of mine within a month. Also, makes sure you use Denso or NGK and steer a wide path around Bosch.

[duffyatkinson]

Originally Posted by sraney

Duffy--since you're kind enough to read our comments and provide your learned responses, here's a follow-on to this one. As I understand it, the forward sensors are used to help calibrate the operating mixture, and the rear sensors (unheated?) are there simply to monitor the health of the catalytic converter(s). However, from this gentleman's experience (and from your previous comments about probably ending up replacing all the sensors to finally stop the codes), it appears that there is some kind of comparison testing not only side to side but from front to rear, even though the rear ones aren't directly involved in the mixture calculation. Any thoughts? Thanks.

OK, I'll give it a shot. Bear with me here, I'm not the definitive source on all the functions... There are a lot of moving pieces to the O2/ECU relationship.

To address your specific question, I don't have enough detailed information about the logic structure of the ECU to state for a fact as to whether there exists a cross-comparison protocol (side-to-side and/or front-to-back), that will throw one of the codes we usually see for a probe issue (failure, i.e. shorts in the system). Maybe one of the members with more experience working with the ECU can weigh in here.

The cascading failure effect we are speaking of here (the P013X and P015X codes) comes from anecdotal evidence only and does appear to be slightly more prevalent side-to-side. It IS interesting that the P003X and P005X codes (sensor heater circuit shorts/malfunctions) don't pop up anywhere near as often as the actual sensor failures themselves.

Remember that the probes are very simple systems. As they can only measure group variance changes (hence the name "lambda" probe...) to the mixture flowing by the sensor NOT a specific, accurate fuel/air analysis, they really only report back to the ECU whether the system is running rich or lean. Also, as they are, in essence, "consumable" parts, i.e the ceramic media degenerates over time (like battery plates), it is reasonable to assume that if they are installed at the same time, when one goes the others are not far behind. Think of it this way: you don't r/r only the worst spark plug in the group; you do the whole shebang, right?

Note that for the sensor units to transmit validated information (voltages), they must be brought up to their operating temp (approx 1500F) quickly, hence the 3rd or 4th wires. For safe engine operation, the ECU can't simply wait for the exhaust stream to heat the sensors. The benefit of this to us as Tundra owners is that our heated units last quite a bit longer due to enhanced residue burn-off occuring at the probe.

As you know, the downstream sensors (i.e. our Bank X Sensor 2s) are required in modern vehicles by Federal (EPA) law to monitor emission control "performance", and we would assume that a failure of one of these sensors would automatically mean a cat issue. I have my suspicions. It is my opinion that, unless you are sending all kinds of junk through your engine that would support this assumption, the cat assemblies will last a lot longer than any O2 sensor, but the service writers seem quick to jump on a code from one of these downstream sensors as an easy way to "offer" to replace your cat (for the low, low price of around $1,400). I think the sensor just dies a normal death and they have had so many successes up-selling expensive (and profitable) cat replacements, they simply don't care to look any deeper for causation.

I know of instances wherein an ECU was continually throwing the same code over and over again on a component replaced several times. Obviously, the component wasn't bad (nor was there a short in the wiring...), so the culprit was a testy ECU. Toyota was able, through their proprietary scanner module (I think it is called the 'MasterTech', or something akin to that...) upload a correction to the ECU to back off on the stringency of the data interpretation. Problem solved.

From what I've seen of members posting pics of the old failed sensor units, most appear to be "charred" with the black, sooty deposits indicative of an engine running with a persistently over-rich mixture. I'm not sure if our trucks, as they age, can be more prone to this condition, or it is the way that the vehicles have been operated. Arguments can be made that silicate contamination via improper filtration, as well as owners dumping all kinds of fuel and oil additives which eventually make their way into the combustion stream, are giving the sensors AND the ECU a lot more to deal with in terms of making proper mixture adjustments and simply staying clean enough to do their jobs. Draw your own conclusions.

As additional background, for your reading pleasure I cobbled together information from various online resources. If you don't feel like reading the whole thing, at least take a scan over the parts I highlighted relevant to our discussion.

-----------------------------------------

Automotive applications

Automotive oxygen sensors, also known as O2 sensors, make modern electronic fuel injection and emission control possible. They determine if the air-fuel ratio exiting a gas-combustion engine is rich (with unburnt fuel vapor) or lean (with excess oxygen). Closed-loop feedback-controlled fuel injection varies the fuel injector output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. In addition to improving overall engine operation, they reduce the amounts of both unburnt fuel and oxides of nitrogen from entering the atmosphere.

Information on oxygen concentration is sent to the engine management computer or ECU, which adjusts the mixture to give the engine the best possible fuel economy and lowest possible exhaust emissions. Failure of these sensors, either through normal aging, the use of leaded fuels, or fuel contamination with silicones or silicates, for example, can lead to catalytic converter damage and expensive repairs.

When the engine is under low-load conditions (such as when accelerating gently, or maintaining a constant speed), it is operating in 'closed-loop mode'. This refers to a feedback loop between the fuel injectors and the oxygen sensor, to maintain a proper ratio. If modifications cause the mixture to run lean, there will be a slight increase in fuel economy, but a possible increase in nitrogen oxide emissions, possible misfiring (at ultra-lean mixtures), and slightly higher exhaust gas temperatures. If modifications cause the mixture to run rich, then there will be a slight increase in power, at the risk of overheating and igniting the catalytic converter, while decreasing fuel economy and increasing hydrocarbon emissions.

When an internal combustion engine is under 'high load', the output of the oxygen sensor is ignored, and the engine automatically enriches the mixture. Any changes in the sensor output will be ignored in this state, as are changes from the air flow meter, which might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.

Function

By measuring the proportion of oxygen in the exhaust gas, and by knowing the volume and temperature of the air entering the cylinders (among other things), an ECU can use look-up tables to determine the amount of fuel required to burn at the stoichiometric ratio (14.7:1 air:fuel by mass for gasoline) to ensure complete combustion.

The probe

The sensor element is a ceramic cylinder plated inside and out with porous platinum electrodes; the whole assembly is protected by a metal gauze. It operates by measuring the difference in oxygen between the exhaust gas and the external air, and generates a voltage or changes its resistance depending on the difference between the two. The sensors only work effectively when heated to approximately 800°C (1,472F), so most newer lambda probes have heating elements encased in the ceramic to bring the ceramic tip up to temperature quickly when the exhaust is cold. The probe typically has four wires attached to it: two for the lambda output, and two for the heater power, although some automakers use a common ground for the sensor element and heaters, resulting in three wires. Earlier non-electrically-heated sensors had one or two wires.

Operation of the probe

The zirconium dioxide, or zirconia, lambda sensor's two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V (200 mV) DC represents a lean mixture. That is one where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). A reading of 0.8 V (800 mV) DC represents a rich mixture, one which is high in unburned fuel and low in remaining oxygen. The ideal point is 0.45 V (450 mV) DC; this is where the quantities of air and fuel are in the optimum ratio, called the stoichiometric point, and the exhaust output mainly consists of fully oxidized CO2.

The voltage produced by the sensor is so nonlinear with respect to oxygen concentration that it is impractical for the engine control unit (ECU) to measure intermediate values - it merely registers "lean" or "rich", and periodically adjusts the fuel/air mixture to keep the output of the sensor alternating between these two states. The time period chosen by the ECU to monitor the sensor and adjust the fuel/air mixture creates an inevitable delay, which makes this system less responsive than one using a linear sensor. The shorter the time period, the higher the so-called "cross count" and the more responsive the system. The zirconia sensor is of the 'narrow band' type, referring to the narrow range of fuel/air ratios to which it responds.

Location of the probe in a system

The probe is typically screwed into a threaded hole in the exhaust system, located after the branch manifold of the exhaust system combines, and before the catalytic converter. New vehicles are required to have a sensor before and after the exhaust catalyst to meet U.S. regulations requiring that all emissions components be monitored for failure. Pre and post-catalyst signals are monitored to determine catalyst efficiency. Additionally, some catalyst systems require brief cycles of lean (oxygen-containing) gas to load the catalyst and promote additional oxidation reduction of undesirable exhaust components.

Sensor failures

Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles. Heated sensor lifetime is typically 100,000 miles. Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation and failure occurs due to catalyst depletion, similar to the reason a battery fails. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, the mileage worsens, and then the probe unltimately reports a failure.

Leaded gasoline contaminates the oxygen sensors and catalytic converters. Most oxygen sensors are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 15,000 miles depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty.

Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealings and greases) or silicates (used as corrosion inhibitors in some antifreezes). In this case, the deposits on the sensor are colored between shiny white and grainy light gray.

Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response. An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel rationing system. Applying an external voltage to the zirconia sensors, e.g. by checking them with some types of ohmmeter, may also damage them.

Hope this helps.

Duffy

[rotarex]

you took to long to change it and all the rest of them goes for shure

[sraney]

Holy moley! Duffy, you are da man. Thanks. I'll be interested to see whether the downstream sensors start throwing codes once I replace the remaining upstream sensor this week....

[duffyatkinson]

Let us know what you find.

[Mike McKinney]

Great thread, I'll give you guys a quick description of what I've dealt with for years. When I bought my truck I had the guys at SOS Performance in Panama City, FL install a custom exhaust. They went with TRD headers, Borla cat-back, and Magnaflow cats. They said from their research that this gave the best performance. Since then I've had constant issues with MILs, most notably, 420/430 codes, which are now constant. They never could figure out the problem, nor could two other exhaust places I took it to, it boils down to the fact that the Magnaflow cats are too high-flow for the Toyota ECU.

I've constantly monitored the engine for years now with no undue problems that I can tell. I changed one of the rear sensors to no avail. I purchased a set of MIL eliminators but haven't installed them, hoping to find a better fix. I even called Magnaflow and spoke with one of their techs but he was stumped also.

I've done a ton of research but unfortunately haven't found a solution except for to replace the cats and go back to factory ones, not wanting to for cost reasons. Anybody have any input to my dilemma? I think in about six months I'm gonna have to do something since I'm in a different area now that requires emissions testing. Any help would be appreciated.

[duffyatkinson]

Originally Posted by Mike McKinney

Great thread, I'll give you guys a quick description of what I've dealt with for years. When I bought my truck I had the guys at SOS Performance in Panama City, FL install a custom exhaust. They went with TRD headers, Borla cat-back, and Magnaflow cats. They said from their research that this gave the best performance. Since then I've had constant issues with MILs, most notably, 420/430 codes, which are now constant. They never could figure out the problem, nor could two other exhaust places I took it to, it boils down to the fact that the Magnaflow cats are too high-flow for the Toyota ECU.

I've constantly monitored the engine for years now with no undue problems that I can tell. I changed one of the rear sensors to no avail. I purchased a set of MIL eliminators but haven't installed them, hoping to find a better fix. I even called Magnaflow and spoke with one of their techs but he was stumped also.

I've done a ton of research but unfortunately haven't found a solution except for to replace the cats and go back to factory ones, not wanting to for cost reasons. Anybody have any input to my dilemma? I think in about six months I'm gonna have to do something since I'm in a different area now that requires emissions testing. Any help would be appreciated.

A couple of questions.

Q: Are you sure the MF cats are correct for your truck, and if so, are you sure that they're failing?

Q: How did you determine that it is a "flow" issue with the cats?

Q: Have the rear sensors been swapped out to see if they're reading right?

Q: And, are you sure that you don't have any type of exhaust leak between the headers and the cats?

[Mike McKinney]

Thanks for the reply, from reading your posts, your knowledge is exactly what I need. Here are my answers:

1. When I spoke with the MF tech on the phone I passed on the model numbers and he stated that they were correct and shouldn't have caused a problem.

2. The flow issue comes from talking with several folks, the MF guy, and a couple of exhaust folks. It's not a definite answer by any means but that's what most of them said, especially because they said the OEM cats are so efficient and the tolerance with other cats is small.

3. I've only changed out one rear sensor with no luck. After reading one of your posts that may be my next step, just change out all four.

4. I had it checked over for leaks once and he did find a few minor ones but haven't checked since.

I realize that there still could be an issue with leaks, bad sensors, wrong cats, but don't know how to definitively troubleshoot it without spending a fortune. It's also a fact of finding an exhaust specialist that I trust to really give it a good look-see. Thanks in advance for anymore help you can give me. I'm really hoping that it's a simple fix and I won't have to change out the cats or something crazy like that.

[duffyatkinson]

Originally Posted by Mike McKinney

Thanks for the reply, from reading your posts, your knowledge is exactly what I need. Here are my answers:

1. When I spoke with the MF tech on the phone I passed on the model numbers and he stated that they were correct and shouldn't have caused a problem.

2. The flow issue comes from talking with several folks, the MF guy, and a couple of exhaust folks. It's not a definite answer by any means but that's what most of them said, especially because they said the OEM cats are so efficient and the tolerance with other cats is small.

3. I've only changed out one rear sensor with no luck. After reading one of your posts that may be my next step, just change out all four.

4. I had it checked over for leaks once and he did find a few minor ones but haven't checked since.

I realize that there still could be an issue with leaks, bad sensors, wrong cats, but don't know how to definitively troubleshoot it without spending a fortune. It's also a fact of finding an exhaust specialist that I trust to really give it a good look-see. Thanks in advance for anymore help you can give me. I'm really hoping that it's a simple fix and I won't have to change out the cats or something crazy like that.

This is one of those situations that arise when you go so far away from stock that it is damn near impossible to reduce a symptom to a single cause. Is it the new cat(s)? Are the sensors sending data to the ECU that is outside the fuel map? Is it just a wierd leak somewhere?

Based on your narrative, you are receiving both the 4X0 codes, which means that the ECU is reporting combustion byproducts transiting BOTH the cats outside of acceptable ranges. Either both of the cats are structurally failing, unable to process the combustion stream sufficiently for the ECU, or the cause is upstream from the cats, i.e. header seals and/or the header-to-pipe joints.

See what I mean? It's gonna be tough to give you much help from here. If the answer isn't in any of the above places, then getting the exhaust system back to stock (painful, I know, but...) might be the only path you can take.

Good luck. Another example of why so many of us on here preach the benefits of keeping the ECU-related components stock.

[suissevale]

Two months back I replaced the O2 sensor in bank one. I was just informed by Auto Zone that the sensor is gone again. My question, what is the downside of not replacing? I live in FL where vehicle inspections are not required.

Thanks in advance!

[bill3508]

Originally Posted by suissevale

Two months back I replaced the O2 sensor in bank one. I was just informed by Auto Zone that the sensor is gone again. My question, what is the downside of not replacing? I live in FL where vehicle inspections are not required.

Thanks in advance!

If its truly not working then most likely poor performance would be the result. I just replace my two front ones and have not had a problem. The rears normally just report out of spec signals and throw codes.

[duffyatkinson]

Originally Posted by suissevale

Two months back I replaced the O2 sensor in bank one. I was just informed by Auto Zone that the sensor is gone again. My question, what is the downside of not replacing? I live in FL where vehicle inspections are not required.

Thanks in advance!

The Bank 1 sensors provide air-fuel mixture information to your vehicle's ECU. A failing/failed air-fuel ratio will allow the engine to drift from its stoichiometric balance point 14.7:1, overly enrichen the mixture, and ultimately will 1. burn out the rest of your sensors, and 2. damage your catalytic converters. The wrecked cats is where you get hit in the wallet.

What brand of sensor did you replace it with? Get them replaced as soon as possible. It'll cost you about $125 for both fronts and you can do the job yourself. I posted a detailed how-to on the procedure in another post here: http://www.tundrasolutions.com/forums/1197269-post2/

Good luck.