What does the secondary O2 sensor really do?

[shiloh51933]

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

Using an Oscilloscope;
the wave forms before cat should move in a up and down motions(hills/valleys) and after cat the wave form should be more leveled out if the cat is doing it's job. Watching the Voltage on a real time scanner you should see the voltage going up to around .800 volts or 800mV and then down to .200 volts or 200 mV. After cat the voltage should be somewhere in the middle of those numbers with the high/low peaks closer together or kinda like a straight line
Using a digi high impedance voltmeter you connect the red lead to o2 sensor signal wire. With engine running in closed loop the voltage should read over 800mV and lower then 200mV and constantly fluctuate. Post cat o2 behind the cat should read be fairly constant, if the catalytic converter working properly. With that being said, normally the pre cat o2 is more important but the after cat o2 could also fail an emissions inspection(DTC).
Hopefully this info answered your question.

[jmmxc]

but the after cat o2 could also fail an emissions inspection(DTC).
Hopefully this info answered your question.

Yes, you did answer the question and I thank you very much.
As you know in this situation one question leads to another...
My problem is that my cat ( an aftermarket) replaced eight or nine years ago, has split along the weld line between the 2 halves. It is giving a bad smell after the engine reaches operating temperature. So, checking with my laptop scanner, I am getting: "P0422 Main Catalyst Efficiency Below Threshold (Bank 1)", with a comment such as: "at least one sensor is working in closed loop...."
So what do you think the statements are referring to?
Thanks,
jmmxc

[MatDesign84]

Gotta love Tennessee. No State inspection.

[shiloh51933]

but the after cat o2 could also fail an emissions inspection(DTC).
Hopefully this info answered your question.

Yes, you did answer the question and I thank you very much.
As you know in this situation one question leads to another...
My problem is that my cat ( an aftermarket) replaced eight or nine years ago, has split along the weld line between the 2 halves. It is giving a bad smell after the engine reaches operating temperature. So, checking with my laptop scanner, I am getting: "P0422 Main Catalyst Efficiency Below Threshold (Bank 1)", with a comment such as: "at least one sensor is working in closed loop...."
So what do you think the statements are referring to?
Thanks,
jmmxc

Both sensors are on bank1, but I would say both sensors would have to be functioning properly to have the ability to diagnose the main catalyst efficiency. I would test the front/rear sensors separately, but you already know that your cat's welds have separated. Try to repair the cat by re-welding the original weld seam, both sensors are probably fine. To plainly answer your question, your cat isn't doing a good enough job. No biggie, just some welding probably, unless she's all rotted out.

[jmmxc]

Yes, the cat is all rotted out and needs a replacement.
It is safe to say that the cat and the 2 O2 sensors should be replaced at the same time in a situation like this.
Thanks again.
jmmxc

[shiloh51933]

Yes, the cat is all rotted out and needs a replacement.
It is safe to say that the cat and the 2 O2 sensors should be replaced at the same time in a situation like this.
Thanks again.
jmmxc

Replacing everything sounds like a good idea, good wrenching.

[jmmxc]

Gee, thanks for the moral support.
i odered a cat and 2 O2 sensors, and hopefully I will get them installed by next week.
Thanks,
jmmxc

[shiloh51933]

Gee, thanks for the moral support.
i odered a cat and 2 O2 sensors, and hopefully I will get them installed by next week.
Thanks,
jmmxc

Yeah, I just replaced the rear(secondary) o2 last week, the primary(front) o2 was done about nine months ago. Both sensors required a blow torch for removal, that and some liquid wrench before and after I heated up with torch. My oem cat has like 97,000 miles on her and is still functional, same goes for rest of cat back(crossing my fingers not to jinx myself).

[jmmxc]

Hi guys, I wish the best of the Holidays to all of you.
Back on this topic from a few vacation days...

Using an Oscilloscope;
the wave forms before cat should move in a up and down motions(hills/valleys) and after cat the wave form should be more leveled out if the cat is doing it's job. Watching the Voltage on a real time scanner you should see the voltage going up to around .800 volts or 800mV and then down to .200 volts or 200 mV. After cat the voltage should be somewhere in the middle of those numbers with the high/low peaks closer together or kinda like a straight line
Using a digi high impedance voltmeter you connect the red lead to o2 sensor signal wire.

Quick questions on testing the O2 sensors:
Do you suggest disconnecting the sensor harness temporarily to check for voltage or wave forms while the car is running.
Or do you have a way to connect the meter or scope probes (somewhere) on the harness.
Do you know if there is some junction connector under the dash for the end connector of the O2 sensors cable harness, that routes the signals to the computer ?
Thanks,
jmmxc

[pinky33]

Hey guys, I have a question to add to this thread.

I recently built a new exhaust for my 97' 850 NA. Stock 5 into 1 header, all 2.5" piping, Vibrant metal cat, resonator, magnaflow strait through muffler. Because of the larger piping and myself not measuring correctly the rear o2 sensor hit the heat guard. I then cut a small hole in heat guard and put a rubber cover around hole in guard to protect o2 wires from rubbing sharp edge. The rear o2 ended up rubbing and breaking. So now i am thinking about putting on one of these.

I would also want to short/trick the ecu into thinking the rear o2 was OK. Is there a way to do this.


Thank you.