Tech question about O2 sensors & catalytic converters:
 

On modern cars, like my '99 996, does the pre-cat O2 sensor affect mixture all through the rpm range? (Other than WOT)? Like most new engines, it has two sets of sensors, pre-cat and after. I've been told that the after-cat ones don't affect performance, they only monitor the cats. So it's the pre-cat ones I'm wondering about.

Car runs great but might be a little down on power/torque. It's the only one I've ever driven, so I would not know until a buddy drove it who has driven many and thought it was not at 100%. The car has super-high miles, (205k), but the motor is newer. (Unknown miles). I'm thinking that they re-used all of the exhaust parts and unfortunately I did not get records showing previous maintenance. It runs really well but I'd love to get a few more easy ponies out of it if they're sitting on the table, so to speak.

On a related note, we suspect that the cats might be original and not operating at peak efficiency. Any way to tell? There are no CEL lights on, car starts and runs flawlessly, etc..., but if they are original they are really old.

Before I spring for some low-mile cats and new O2 sensors, I thought I'd run it past you guys. I think that this is a generic question, IOW, if you have experience w/ some other modern car, please chime in. TIA as always! :cool:

I'm going through this with my Corolla. The secondary O2 sensor apparently does what you said. Sigh, I am going to replace the sensor first. A cat is over $1200.-.

The sensor is supposed to trip the CEL at about 50% efficiency.

From a Jeep Forum for additional "generic" information that may not apply to high dollar vehicles.

http://www.jeepforum.com/forum/showthread.php?t=508460

John_AZ
1988 924S 58K to Show
1987 924S 112K to Go DD
1998 Cherokee to Tow

can't wait to hear the experts chime in, I was wondering about this too

My understanding is that the cat doesn't affect *performance* because there is nothing measuring its operation & output. It cleans up the emissions, but doesn't give any feedback into the ECU to affect driveability or power.

The exception is if it's plugged and causing excessive backpressure, but that is a mechanical issue, not a sensor/computer one.

Mind you, this is my understanding based on 10+ yr old cars that I've owned. Probably the newer, more complex cars measure post-cat emissions

I also would like to know. I put a bad tank of gas in my Tahoe not too long ago, and the pre cat O2 sensor kicked on the CEL light. It ran like crap, and I was WAY down on power which sucked since we were camping, and I was pulling my camper.

I ran the tank to near empty, put good gas into it, and the CEL went off, and it ran great.

Bill

I want to hear some replies also but

My understanding is

the second O2 sensor monitors the catalyst efficiency or the ability of the converter to store oxygen

The primary (in this case wide band) is active (fuel system in closed loop control) except at WOT and warm up.

I realize that they get plugged and can affect performance, my question was "how can you tell when it's getting plugged?" It might be easy to tell if one is completely plugged, but how about only partially? I'm thinking that they do not plug-up all at once, that it's a gradual process. A partially plugged car would simply rob you of power and efficiency, w/o any blatant symptoms.

The O2 sensor question is a separate issue. What will a bad sensor (or 2) do? Do they affect mixture all through the rpm range, such as under light acceleration?

New cats, or even used ones, are very expensive and not the kind of thing that I would change just for the *why not* of it. Same w/ sensors, I'd need minimum (2) @ ~$150 each. I need to know how to diagnose them.

FWIW, slodave, new cats are out of the question for me. I found a set of super-low mileage used ones at a Porsche salvage for about $300/set, as opposed to ~$1k each brand new. It is not a part that wears quickly, they often last 200k miles or more. Good luck. :cool:

Bad gas probably didn't do anything to the cats, unless the fuel contained something that kills cat converters. Most likely it had H2o in it. Your vehicle is most likely fine, by that story. I hope so, but I'd bet on it. :)

When the cat plugged on my '96 jetta, I could barely see through it when I held it against the light like an air filter. I'm guessing you're not looking to take yours off though. Pressure gauges, I guess.

I've seen manuals that show a pressure gauges before & after the cat. That makes sense.

When the O2 sensor was bad on dad's Carrera, it ran rich ALL THE TIME. Gassy idle and even smelled rich underway, if you caught a whiff of the exhaust (cabrio). The Bently has a set of voltage values it should giving with the engine warm, it was way off. Took 15-20 mins to test, and another 15 to change it.

I'm sure the same info would be out there for the 996

Yeah Denis, I didn't think it did anything to the cats. I figured it was water in the gas.\

The high price of the cats (the metal in them) is a reason we are seeing a lot of them being stolen right now.

Bill

ahem..a word from someone who sells cats for a living...your corolla has a program flash at the dealer to "eliminate" the rear o2 senser..and fix persistent
p0420 codes

second the role of the pre cat o2 senser is the same as it always was..it is the mixture regulation control...the after cat sensers are "nanny" devices that ONLY moniter catalyst efficency...they play no role in mixture unless you have a VERY late model car(even then it is really only long term fuel trim)

a properly functioning modern cat has little to no effect on hp, the big thing to understand is that they DON"T wear out..they get destroyed by other upstream issues. They are also often replaced by people who do not understand that a p0420 code for inefficency is not alway a bad cat...alot of times it is a vacuum leak, carbon deposits on the backsides of valves(effects mixture to extreme), a misadjusted throttle..a mad MAF or other things that don't throw a code. Basic diagnostic ability is severly in need these days and many if not most mechanics are just parts changers. I get lots of calls on bad cats..i ask specific pointed questions(i know the answers) and 99.9% of the time these "experts" have no idea what i am talking about.
good example is honda oddessy. these are NOTORIOUS for bad cats...and they eat oem/aftermarket ones up it seems. well if the mechanics go only by the obd codes, they are treating the symptoms...not curing the disease. The problem is plugged egr ports, there is no code for this as it is only a code for open or malfunctioning egr valve...not for low flow(the code says low flow when it is stuck shut...but never if it actually opens and closes fine btw) but almost all mechanics will condem a cat and not think to diagnosis this. Most "techs" don't even know that if you run the cat monitor, and it passes, the cat is good and the problem lies elsewhere.
A GOOD tech will diagnose the problem in conjunction with scanning codes..not just to charge extra, but to ensure that the correct repair is done
a very good tech article on cats is on troublecodes.net

Even without a CEL light, I would check for stored codes. Not all of them will trigger the light. You could have vacuum leaks or other minor issues than can affect performance like old plug wires that leak voltage at higher rpms.

Of course, this may be a head game and your buddy is laughing his ass off now. ;) Probably cheapest to put it on a dyno and check numbers before guessing at the (alleged) problem.

With that many miles, I'd spring for new sensors, unless you know for certain they have been changed. They do go bad over time.

Keep looking around for cats if you need one. A generic cat should be less than 2 bills. They don't last as long but will get you thru the inspections.

Do I just go to the dealer and asked them to flash the computer? Fill me in... Thanks! I have to smog the car in January. A mechanic friend did notice that the Corolla is running a little rich...

Good thread, thanks for all the info. Nick, are you saying that modern cats do not gradually wear-out or plug-up and lose efficiency? That's the way I understood it, just confirming. Thanks again.

NICKG is right on the money ( he ought to be, he does it or a living, right?).

If you don't have a OBD scan tool with real - time engine, and sensor monitoring, everything is little better than a guess. With a scan tool you can look at all four O2 sensors, and monitor the activity. Post-cat sensors will have an even, slow voltage stream, but they should show fluctuations in voltage. A pre-cat sensor should react quickly, and be all over the place voltage wise once the car warms up and enters closed loop, unless you have a perfect engine that has all cyliders perfectly balanced (I've never seen one).

OBD2 warm up, and actual temperature warm up are two totally different things. Part of the OBD2 protocol, is that the o2 sensor has to be warm, measuring the exhaust stream, and in closed loop in a preset time from the first time that engine fires and runs. This is why we have heated O2 sensors.

I don't think cats will go bad for the lifetime of a car IF it's kept in tune, and you run good fuel in it. Sulphur, and constant exposure to trash gas is usually what gets them. At a salvage yard my friend owns they have a yard truck that ONLY gets gas from incoming totals. These totaled cars may sit in the insurance pool lots for up to a year. We call that "Swamp Gas" The yard truck will run on it, but not well, but it hasn't killed a cat yet in 3 or 4 years, so they are more robust than people think.

NICKG, flat time is killing these techs, and turning them into parts changers, but there are still a few good ones around. Customers scream bloody murder when they get charged two hours for diagnostics.

On a Porsche, the O2 is normally ignored at WOT, and also above 4500 rpm. Haven't confirmed the >4500 on a 996, but it should be no different, as you don't want to going stoichiometric above that.

As for cats clogging, they shouldn't do that unless your motor burns excessive oil, ran too rich, or got too hot from raw fuel.

I've heard that you can measure whether a cat is functioning properly by aiming a laser pyrometer at the inlet and exit side of it, there should be a certain temp difference(?) Anyone know about this?

Also, since we have some good technical brains on this thread, are those spray cans they sell to clean your MAF worthwhile? If so, I thought it might not be a bad idea with my high miles, worst case is it does nothing. Do they need cleaning? TIA.

Do not clean hot-wire MAF with anything. MAF wires get heated up red-hot after each shutdown (trough Motronic) in order to burn off eventual filth/deposits. Eventual cleaning chemicals deposits might make wire go off.