What does the secondary O2 sensor really do?

[shiloh51933]

I always thought and was taught that the secondary o2 or after cat o2 was mainly used to monitor. During one of my engine performance classes at school we had discussed this topic and as I stated , the second 02 after cat is primarily used to monitor. During this discussion my professor had mentioned in certain rare situations the after cat o2 plays a role in the engines performance or lack of performance. I recently replaced the primary o2 sensor(bank1, sensor1) and 6 months later I'm scanning a code (p0136), o2 sensor circuit. After going threw all the possible reasons for this DTC, I came to believe that my OBD2 scanner wasn't giving me all the info i needed. I was right and was able to get a better description of the fault with a far more advanced scanner than my hand held. Turns out the actual fault/issue's source is with the after cat o2 sensor. My 850GLT is running too rich, which I believed initially was due too a defective primary o2 sensor. I couldn't believe that a direct fit Bosch o2 would go bad already, I installed the o2 myself of course and didn't damage the o2 during installation. I went threw my entire ignition system and fuel system looking for something, anything that could cause her to run too rich. Anyway, I'm sure that many of the members here have there own opinions about the function of the after cat o2 sensor and I'm looking foward to reading them.

[Ozark Lee]

The secondary sensor is purely to monitor the efficiency of the catalytic converter and be a tattle tale if the converter isn't up to snuff.

...Lee

[tjts1]

Bosch O2 sensors just like Bosch spark plugs are very hit or miss. The only reason I used one was because it was extremely cheap. I can monitor my O2 sensor voltage and LTFT in real time so I'll know quickly if it ever goes bad. So far so good.

[shiloh51933]

Bosch O2 sensors just like Bosch spark plugs are very hit or miss. The only reason I used one was because it was extremely cheap. I can monitor my O2 sensor voltage and LTFT in real time so I'll know quickly if it ever goes bad. So far so good.

Yesterday I was doing the same thing, monitoring the sensor voltage and fuel trims in real time with a better scanner. My hand held only takes snap shots of live data, so it doesn't really help. I was able to actually watch the voltage go up and down while is was actually happening instead of getting a quick snap shot with my hand held.

[jmmxc]

What type of scanner do you guys use to monitor the O2 sensors?
I am using the Elmscan 5 Compact, a multiprotocol ODB Scan tool. It is a USB device that I connect to my laptop.
I get live data as well about the activities of the sensors, I am not sure what the info regarding the O2 sensors mean at this point. I have a cat and possibly sensor or sensors issues. As I am about to change my cat (because the top and bottom split open) I would like to determine if my sensors are good because they are expensive to replace.
Any info would be good.
Thanks,
jmmxc

[modabasi]

My 98 S70 T5 Euro does not and never had a secondary O2 sensor, I've replaced the Cat with a free flow silencer and the performance boost was incredible. Bosch website says that secondary O2 sensors are there to monitor if the Cat is doing good or not, I don't think it has anything to do with fuel/air mix calculations or anything.

[jblackburn]

It doesn't take the rear sensor into account for fuel/air calculations until the 1999 engine management system.

The newer models, to some extent, take the rear reading into consideration for making sure the front one is also in check with its readings.

[jmmxc]

Are we saying that the critical unit is the upstream O2 sensor for emissions inspection?
So, what are the clear indications (based on scanner data) of a bad cat, and by extension a failing aft O2 sensor?

jmmxc

[shiloh51933]

I always thought the rear/secondary o2 was to monitor (with a few exceptions), but i didn't think my '96 glt was one of those exceptions. I know I'm going to get some disputers on this but this is what it appears to be. My primary o2 is a Bosch direct fit that's new and I've gone threw everything a couple of times looking for another reason she would run too rich. The DTC reads O2 sensor circuit bank1 sensor2, I believe the code is P0136. The primary o2 voltage is going up/down like it should, no exhaust leaks, no vacuums leaks, ignition system checks out, fuel system checks out. I had to finally bring the car to school to have one of my professors double check everything and this is what were coming up with. I know it shouldn't interfere with the performance of the engine but it's the only thing at fault. I know I won't be sure until I replace the secondary o2 sensor.

[shiloh51933]

Believe it or not, changing out my secondary o2 straightened her out. She performs as she should and I'm still surprised to say the least. Like I said earlier, I could find nothing else wrong with her and the primary o2 is less than a year old.