Post-cat O2 sensors

[Frodo]

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?

[Frodo]

Come on, guys...SOMEONE must have some sage advice on this topic. Speak up!

[landrovered]

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.

[gschotland]

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).

[Frodo]

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!

[boxzilla]

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.