Two Catalytic Converters - O2 Sensor sits between them - Upstream is clogged - Test Pipes?

I bought a 2010 Prius as a beater. It gets nearly 4x the gas mileage as my truck, and I care less about it than my wife's newer car. I have a few hundred bucks into it, and I've been driving it regularly. I'll call that a win.

Back in my garage days (I worked doing batteries, tires, exhaust work, oil changes, etc during high school and college) we thought test pipes were a poor man's header, and we'd put them on all our (other youngsters who worked at the same shop) cars. Mind you, that was 20 years ago now, and that state didn't have emissions testing.

My current county doesn't do emissions testing either. In fact, we don't do any type of road-worthiness inspections. A cheap car buyers dream.

Anyway, my cheap Prius has been throwing codes for catalytic converter efficiency. Not surprising considering how much oil this thing has burnt in it's 214,000 mile life. The 2010-2014 models were noted for excessive oil blow-by due to piston ring issues. This in turned gummed up the EGR cooler, created misfire issues, head gasket issues, and lastly, catalytic converter clogging.

My thought is to remove the upstream catalytic converter. More than likely this one is the problem part, and leave in place the secondary cat. So the O2 sensor would be getting unfiltered, un...catted flow.

Does anyone know the best practices for placement of O2 Sensors when installing aftermarket high-flow cats? Is there any benefit, or requirement to putting them upstream or downstream of the catalytic converter? Will my car end up in a firey hell for being a mildly more polluting Prius?

I'd try sticking the rear O2 after the rearmost cat and seeing what happens.

If the front cat is upgeberked, the rear cat is most likely plugged too, from debris blowing downstream.  Have seen this happen on mutiple vehicles where a front cat broke up and the brick went sideways - the cells in the rear cat got filled up by the powdered remains of the front cat as it beat itself to death.  Just sayin'.

In this case, I don't expect the debris to be flowing much, that's part of the problem. Others have actually removed the exhaust and found that the secondary cat was clean as a whistle, but the primary was clogged up at the front. 

My question is, does the O2 sensor know that it's got a cat in front of it? Is looking for cleanish air? Or just air of a certain temp?

In reply to pheller :

Generally speaking, it's checking to make sure that the exhaust stream has been converted.  It should be detecting no or minimal oxygen at any time the engine is running stoich.

So is that to say most o2 sensors don't really care about the existence of the catalytic converter, they are just looking to make sure the burn is complete?

They're looking for a differential between the two O2 sensor readings.  If it's large enough, they're happy.

pheller said:

So is that to say most o2 sensors don't really care about the existence of the catalytic converter, they are just looking to make sure the burn is complete?

Sort of?  That's an oversimplified way of looking at it.

The catalytic converter's presence is to make sure the burn is complete, if the engine could do that by itself then the converter would not be needed.

There used to be devices sold that would send a fake .650v signal to the PCM to make it look like the cat was working.  The problem is, the PCM also has diagnostic routines for the rear O2 and will watch the rear O2 to make sure it is switching or otherwise following the front O2 before the converter lights off, and in many cars the PCM will periodically deliberately drive the engine rich or lean to make sure the O2 sensors can report that.  So a rear O2 signal is pretty hard to fake compared to just putting a functional converter in front of it.

In reply to Knurled. :

To add to your last statement, this Prius probably has a WB sensor in front of the first catalyst that does most of the work, but that second sensor is required to move, or it will certainly throw a code.  Toyota has been using the second sensor as a control sensor for many year- it does some very minor but important trim- so if it's seeing a bad signal, it will be noted.

Also, for a car of that era, that second catalyst isn't going to have much metal on it, so even if you move the sensor downstream of it, it will probably throw a code.  

Sadly, the most straight forward solution is to replace the front brick with the best one you can afford or want to spend.

Well that was my question: do I even need the front one, from a sensors standpoint? 

The sucky thing is, Toyota did not build these to be cheaply replaced. It's O2 Sensor 1, then cat 1, then o2 sensor 2, then cat 2, then Exhaust Gas Heater thing, then rest of exhaust. No bolt flanges between those pieces either. 

Luckily I can find direct fit replacements for $275, but that's not $50 test pipe cheap, either.

The front O2 is the most important one- it does 95% of the control work, so yes, it's required to run the car in a decent way.  The second one is a very important trimming one.

And I didn't know that Toyota was using exhaust heat to heat the car up for that model.  I knew it came later- just not that early.

On top of the complexity, since the Prius is PZEV, it also has a pretty darned expensive catalyst in the first place- lots of precious metals on them- probably a lot of palladium or platinum (by then, it was one or the other) and an amount of rhodium- which is currently trading at $4800/oz.  And the first one is where most of the metal is- it does roughly 95-98% of the work.

Again, I wasn't planning on deleteing the O2 sensors, just removing the catalysts between them or after them. Well, completely. Stock O2 sensor placement, just without cats. 

You can try to move the second sensor behind the second brick, but I'm betting that it's low enough loaded that you will still have a MIL light.

And with that package, I'm not sure if you can find space to put a boss in for the sensor....

Flanges can leak, leaks cause catalyst inefficiency or false O2 readings or both compounding on each other, so OEMs use as few as practical as long as they can get it assembled and out the door.

pheller said:

 

Luckily I can find direct fit replacements for $275, but that's not $50 test pipe cheap, either.

It sounds like you know the direction to take. Those cars are absolute jewels of engineering. If it were mine I'd respect it and keep it as close to Toyota designed OEM as I could within reason. 

While $275 isn't test pipe cheap, it is still cheap for what it is and what it gets you.

Driven5 said:

While $275 isn't test pipe cheap, it is still cheap for what it is and what it gets you.

My options for the Volvo wer $1400 for an OEM cat or $800 for aftermarket.

The aftermarket one didn't fit right and the rear O2 fouled on the driveshaft.  An O2 right angle adaptor got me the clearance,  but due to the way the ECM trims fuel with rear O2 data (see alfadriver's posts for hints as to why) my fuel economy has dropped about 20-25%.  (From 20-ish to 16-ish)  I also occasionally throw a P2xxx series code for rear O2 not switching when commanded.

pheller said:

So is that to say most o2 sensors don't really care about the existence of the catalytic converter, they are just looking to make sure the burn is complete?

Not really.  Upstream O2s measure what comes out of the engine to tune the fuel curve.  Downstream O2s detect the after-cat exhaust to make sure the cat is working.  When the catalyst becomes spent, the downstream O2 starts reading things that are very close to upstream and throws the cat efficiency code.

Clogged cats usually don't throw that code.  They get very little information from the downstream O2 and think the catalyst is working even better.

In reply to Curtis73 :

By the time the catalyst gets so hot to actually melt, the metals on the surface will coagulate (so to speak) which will trigger the cat inefficiency code.

(and the downstream sensor is used to trim fuel, too- so having a bad front cat will change that feedback- causing a different fault reaction)

pheller said:

Well that was my question: do I even need the front one, from a sensors standpoint? 

 

The sucky thing is, Toyota did not build these to be cheaply replaced. It's O2 Sensor 1, then cat 1, then o2 sensor 2, then cat 2, then Exhaust Gas Heater thing, then rest of exhaust. No bolt flanges between those pieces either. 

 

 

You need a Cat 1 if you want the CEL to go away.  The second O2 sensor reads that the catalyst is doing catalyzing things.  Putting a test pipe in its place will make the two sensors read the same thing which is what tripped the code in the first place.

Either move the 2nd O2 sensor to behind the 2nd cat, or maybe cut them both out and put Cat 2 in the Cat 1 position.  I doubt it will work, though.  The first cat is the one that's designed to get hot quick.  The second one is used as a last-ditch scrubber to pick up anything that the first one missed.  They're different designs, so I doubt that Cat 2 would be effective enough to fool the O2s

alfadriver said:

In reply to Curtis73 :

By the time the catalyst gets so hot to actually melt, the metals on the surface will coagulate (so to speak) which will trigger the cat inefficiency code.

(and the downstream sensor is used to trim fuel, too- so having a bad front cat will change that feedback- causing a different fault reaction)

Yup.  The platinum coalesces when it reaches a certain temperature.  Once the crystals become a flat glob there isn't enough surface area for the exhaust to catalyze.  But that is different from clogging. 

The downstream O2 (not sure about that prius or that year) doesn't affect the engine's fuel trim in major ways.  Whereas the upstream O2 alters fuel in the map up to about 25% leaner or richer, the downstream makes tiny modifications on the order of 1-2% to alter the emissions.  If the ECM notices oxygen creeping up a hair at the second O2, it can fine tune things, but in the grand scheme I wouldn't say it is used the tune the fuel curve.  Not like the upstream sensor does.  The upstream sensor tunes the fuel curve like any other OBD2 vehicle.  On most vehicles, the downstream sensor is just there to check catalyst efficiency.  In some vehicles, the ECM is programmed to fine tune the emissions profile with information it gets from downstream, but it's just that.  Very fine tuning.

Curtis73 said:The downstream O2 (not sure about that prius or that year) doesn't affect the engine's fuel trim in major ways. 

Oh yes it does.

It isn't talked about in official literature much, but the rear O2 affects fuel trim in two major ways.  The first way is in calculating how much the front O2 sensor has skewed due to age, by comparing the two sensors in the brief period on cold startup before the catalyst lights off.  A rear O2 that is failing can cause a front O2 fault or fuel trims way out of whack because the programming generally assumes that the rear O2 is okay and the front O2 is bad, because the front O2 sees the brunt of exhaust temperatures.  But if it isn't that bad YET then any miscalculated skew will result in wildly out of whack fuel trims.  And, because an engine not running stoich will result in the cat not lighting off, a catalyst inefficiency code.

The second way is that some PCM control algorithms actually look at the rear O2 for some fuel control, not the front.  The reason for fuel control is to keep the cat working after all, so the focus should be on the rear O2.  A functioning cat with the engine at stoich should see the rear O2 hovering steady at .600-.700v.  A dead cat would see the rear O2 switching... unless the engine was running rich all the time.  And of course when the computer has to dump fuel to make the rear O2 read happily, but the front O2 is reading pig rich, for some vehicles that will result in a fault code... for front O2 stuck rich!

Need to use one's brain when diagnosing cars, cannot just take a code a face value.

In reply to Curtis73 :

The fault correction depends on who calibrated it.  If it were me, seeing the fault, I would trim it to not cause a big problem.  But I have seen others fault it so bad that it would force some really crappy fuel economy, which would force the owner to get it fixed.

You can call it a minor trim, but having been doing that specific work for so many years, i would not.  It's a key part of getting modern super low emissions.  And I know by now- since around 2000, all cars use the second sensor to trim the fuel- as doing that saves a ton of money on catalyst materials if you don't.

Then again, I'm not really sure what you mean by "fuel curve"....  Maybe for aftermarket systems, there's a curve, but I try for specific patterns in the sensors to  provide the best performance.

Knurled. said:

The second way is that some PCM control algorithms actually look at the rear O2 for some fuel control, not the front.  The reason for fuel control is to keep the cat working after all, so the focus should be on the rear O2.  A functioning cat with the engine at stoich should see the rear O2 hovering steady at .600-.700v.  A dead cat would see the rear O2 switching... unless the engine was running rich all the time.  And of course when the computer has to dump fuel to make the rear O2 read happily, but the front O2 is reading pig rich, for some vehicles that will result in a fault code... for front O2 stuck rich!

 

I've wanted to fill this gap in for a while- and I keep trying to tell people this.... but the reality of the situation is that the second (and sometimes third) O2 sensor does not actually measure O2- as there's just none there to detect.  If there's actual O2 there to measure, either the injectors have been off, or there's a leak.  

The sensor does measure the relative amounts of oxygenates (mostly NO by this point) and reductants (mostly CO, but sometimes H2 and rarely HC).  Just to add some technical trivia to the board.

The front one does the same thing- but the oxidants are O2 and NOx- a pretty decent amouint of O2 in the raw exhaust.  

So how do people get away with adding test pipes to modern cars, some with dual cats from the factory, without tripping CELs? 

Most of the time they are getting a flash/tune to gain performance benefits of a more free flow exhaust, and no mention of needing to do a tune for the removal of the cats and its impact on O2 Sensors. 

If people aren't worried about emissions tests, they also aren't usually worried about check engine lights.