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Old 08-12-2010, 12:40 PM   #1
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Post-cat O2 sensors

I gather that 02 Sensor 2 (the ones downstream from the cats) are basically there to assess whether the cats are doing their jobs or not. If that's the case, if they (the sensors) begin to fail, will it at all affect engine performance?

I ask because I've had an intermittent CEL, the code for which indicates a problem with Sensor 2, Bank 2. Also, last time I had the car in to the shop the guy said his PIWIS scanner was indicating the Sensor 2 for the other bank was 'getting lazy'. The car runs pretty well but, it seems to me, doesn't seem quite as responsive on hard acceleration as previously.

I had planned on replacing both post-cat sensors, hence my question, ie: Can I reasonably expect any improvement in engine performance?
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Old 08-13-2010, 03:44 AM   #2
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Come on, guys...SOMEONE must have some sage advice on this topic. Speak up!
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Old 08-13-2010, 04:46 AM   #3
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The answer is...it depends.

It depends on which error code(s) is being thrown. If you have the "slow response" code then no, If you have "failure" codes then the DME will kick the fuel curves to a more moderate setting (like limp home but not as bad) and you will not get the performance or economy that you would with the codes cleared.

My 3.6 has a tweaked DME and the difference in the curves is easily noticed, in a stock set up it may be less obvious but it operates the same.

So the answer is, replace your secondary O2 sensors if they are failing and then you do not have to suffer the issue.

Simply clearing the codes will give you a change in performance until they come back on.
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Old 08-13-2010, 05:12 AM   #4
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At 64k miles one O2 sensor went and it was replaced. At 70k at 2nd one started giving me intermittent CELs. I read here that it's best to change all 4 at once, so I changed the remaining three even though the car was running fine, and experienced a noticeable improvement overall in driveability.

O2 sensors tend to fail around the same time. If you're planning to hang onto your car for a while, best to just change them and get it over with, or they're likely to come back and haunt you at an inopportune time. I got them with the OEM type connectors from Maxspeed Motorsports for $99 each, which was the best deal out there at the time (last fall).
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Old 08-13-2010, 06:51 AM   #5
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Thank you.

The code was P 0159 which, I believe, is indicative of a sluggish response (vs a complete failure).

It's funny, some places I've read about replacing ALL of the sensors like every 30K miles...other places people have said they're supposed to last like 100K (I know this really is not very likely) and that one should only spend the bucks to replace the ones throwing codes and leave the rest.

Seems to be a lot of differing opinions out there!
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Old 08-13-2010, 07:31 AM   #6
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Frodo ~ I agree, its a very personal decision. I replaced my 2 post cat units even though only 1 had thrown a sluggish code. When 1 of my pre-cat units threw a code, about 1 month later, I only replaced the 1 unit. The other pre-cat unit is still going strong.

If you have the $$ to replace all 4 and you are paying someone else to do it for you, then probably makes sense to pay for 1 install and get it done. Since I do my own wrenching, and most times you can find them for sale with free shipping...then why not just replace them as needed? I don't worry about running the car for a week while waiting for them to arrive as they only impede my mileage and pose no serious threat.

I was proactive and did replace my AOS at 70k since I was the second owner and did not have a service history, and feared the negative impact of a failed AOS was more significant than I wanted to experience. ounce of prevention vs. pound of cure scenario.
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Old 08-13-2010, 07:37 AM   #7
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I will put this out there for your consideration.

The pre cat sensors do the heavy lifting for the fuel injection system, they set the overall rich/lean mixture for the engine. If a pre-cat sensor was to fail on my car I would replace both.

The post cat sensors are only to confirm that the cats are working. They are not nearly as important as the pre cat sensors. I would replace them one at the time.

Also the only thing that makes it a "Porsche" O2 sensor is the plug (connector). They are bog standard Bosch thimble type O2 sensors and if you are willing to do some soldering you can buy them for $39 but you have to solder your Porsche plug to the wires.
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Old 08-13-2010, 07:52 AM   #8
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Quote:
Originally Posted by landrovered
I will put this out there for your consideration.

The pre cat sensors do the heavy lifting for the fuel injection system, they set the overall rich/lean mixture for the engine. If a pre-cat sensor was to fail on my car I would replace both.

The post cat sensors are only to confirm that the cats are working. They are not nearly as important as the pre cat sensors. I would replace them one at the time.

Also the only thing that makes it a "Porsche" O2 sensor is the plug (connector). They are bog standard Bosch thimble type O2 sensors and if you are willing to do some soldering you can buy them for $39 but you have to solder your Porsche plug to the wires.
Makes sense.

I thought about doing the standard/generic route but got lazy and ordered the plug-and-go ones, though I am planning on putting them in myself. It's not too tough, is it? I don't have the special 22 mm wrench...how hard is it if one uses an adjustable wrench??

BTW, I've read that you should crimp NOT solder. Don't know why...or even if it's truly an issue.

Last edited by Frodo; 08-13-2010 at 07:54 AM.
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Old 08-13-2010, 08:15 AM   #9
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It does not require a special tool, it is butt simple as long as you remember to unplug the sensor before you try to remove it. Also - DO NOT PUT ANTISIEZE OR SEALER on the sensor when you re-install, the chemicals can contaminate and ruin the sensor.
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Old 08-13-2010, 12:02 PM   #10
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It is not a difficult job...the only 1 i had trouble with was the post-cat, passenger side ..... it's a tight spot and the wire and head unit have to find their way around a number of obstacles.
I would recommend getting the right wrench as it may be frustrating to do otherwise. Try Harbor Freight is you have one local, or since it is only 1 wrench just go to Sears.
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Old 08-13-2010, 01:42 PM   #11
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Just finished. Pretty easy. Crescent wrench worked fine, BTW.
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Old 08-13-2010, 04:01 PM   #12
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How many oxygen sensors does a 97 boxster have? .... qty4 I assume....right?

How many wires do they all have coming out of them?

I found some universal ones for $20-$30 but not sure which ones I need specifically.

link:
http://www.thepartsbin.com/catalog/?N=4294967254&Nr=OR%28AND%28make:Porsche,model:Boxster,year:1997 %29,AND%28universal:1%29%29&Vi=1698+11567+4294963368&y=1997&mk=Porsche&md=Boxster
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Old 08-13-2010, 05:26 PM   #13
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Quote:
Originally Posted by dirkdiggler
How many oxygen sensors does a 97 boxster have? .... qty4 I assume....right?

How many wires do they all have coming out of them?

I found some universal ones for $20-$30 but not sure which ones I need specifically.

link:
http://www.thepartsbin.com/catalog/?N=4294967254&Nr=OR%28AND%28make:Porsche,model:Boxster,year:1997 %29,AND%28universal:1%29%29&Vi=1698+11567+4294963368&y=1997&mk=Porsche&md=Boxster
I believe the 1997 has four as well.

There are four wires coming out of each.

Here's some links that might be helpful:


http://www.pelicanparts.com/techarticles/Boxster_Tech/19-FUEL-02_Sensor/19-FUEL-02_Sensor.htm

http://sites.google.com/site/mikefocke2/oxygensensors

You can also just do a "Search" for oxygen sensor on this forum.
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Old 08-14-2010, 06:56 AM   #14
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WOW. I just took her for a 15-20 mile down the highway with the new sensors installed. I really didn't think replacing just the post-cat O2 sensors would make much of a difference in performance, but WOW. The subtle hesitation (and it was subtle...easily ignored) is gone. She was chompin' at the bit, just raring to go, and go she did. Had to watch myself (as well as watching for brake lights in the distance!)
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Old 08-16-2010, 05:08 PM   #15
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so..... the cheapo $30 on Partsbin.com can be spliced into a 1997 boxster for sure?

I was reading stuff about some hollow tube around the wires that cannot be blocked or something?.....or does this not apply to the older boxsters

How many people have used the O2 sensors that need to be spliced in with good results?
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Old 08-17-2010, 04:19 AM   #16
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If the O2 sensor is a generic Bosch four wire thimble type O2 sensor then yes it can be spliced in.
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Old 08-17-2010, 07:22 AM   #17
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only Bosch Brand??
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Old 08-17-2010, 07:48 AM   #18
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It must be Bosch type, personally I would insist on Bosch brand.
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Old 08-17-2010, 02:51 PM   #19
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what makes the universal Bosch ones different from the other cheap universal O2 sensors?
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Old 08-25-2010, 07:23 PM   #20
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anyone know the difference?? related to my question directly above
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