Dual O2 Sensors and how they work?

[EagleMark]

Here's one that I'm not sure about and may change with systems?

Is the post cat O2 sensor supposed to switch? Or have steady reading?

[RobertISaar]

pre vs post cat O2 sensor signals. the post-cat is simulated, but that's about what you could expect to see with a well functioning system.

[daleulan]

What you see out of the postcatalyst sensor can depend on the design OSC (cerium dioxide) loading of the catalyst. Early OBD-II catalysts used lighter OSC loading and so you would see an attenutated oscillation out of the rear sensor like above. Newer low-emissions catalysts have very high OSC loading and another interesting algorithm that I forgot about. The generaly target of most late model controllers is to maintain the postcatalyst sensor at a very steady target voltage - for gasoline it is commonly about 0.55 to 0.6 volts but may vary based on load and speed. Definitely the target changes with the fuel in use and also the NOx production of the engine. So on newer controllers you should see the postcatalyst sensor stay very steady. After a deceleration the voltage should return to steady-state quite quickly - within 5 seconds or so, maybe even quicker. You should see an enrichment in the front catalyst. The other sneaky algorithm is one that monitors the catalyst oxygen storage capacity and adjusts the front O2 sensor step-and-ramp to ensure that the postcatalyst sensor doesn't wiggle much.

[Playtoy_18]

Does anyody have any insights into the single bank front 02 on fwd 3800's versus the dual banks on the rwd's?

[Hog]

Playtoy is the FWD kinda like the Vortec 4.3/305?
I notice on the Vortec 4.3 L35 and Vortec 305 L30 that they use a 3 Precat O2 sensors, 1 for Bank 1, 1 for Bank 2 and then a 3rd O2 sesnor which reads BOTH Bank 1 and 2 mixed together just forward of the cat, then the aftercat O2 sensor.
The Vortec 350 L31 and Vortec 454L29/L21 use the conventional B1S1, B2S1, B1S2,B2S2 config with 2 precat and 2 postcat.

I remember something from Monodax about the post cat sensors affecting fueling. The acronym was FOASCO or something like that. I really wish we could access that info on Monodax, it was a great resource.

peace
Hog

[EagleMark]

I always wondered what that third O2 sensor was for in Vortec...

From what we have gathered and I partially found in a 98 Vortec Dissasembly the post cat O2 sensors do effect fueling up to 10%. That said I just did a 97 Vortec and both post cat O2 sensors were swinging! Both cats were shot. Disconnect and remove rear O2 sensors from cal and man did it run better.

[Playtoy_18]

No, except for the camaro most 3800's I've seen have a single 02 sensor 1, and single 02 sensor 2.
So it has only a single 02 sensor for fueling both banks and one after cat for monitors.

Most vortecs I've dealt with have 4 02 sensors.
Precat/fueling is bank 1 sensor 1 and bank 2 sensor 1
Postcat/monitor is bank 1 sensor 2 and bank 2 sensor 2

Low emission ulev, pzev etc would have like 3 sensors total for say a 4cyl.
Precat/fueling would be bank 1 sensor 1 then there will be another installed in cat towards the front and another that is post cat.
These would be bank 1 sensors 2 and 3, but not necessarily in order "I THINK".
I suspect sensor 2 might be always post cat monitor and sensor 3 is in the middle designated for whatever makes it a pzev.
Confused me everytime, had to look in book and verify etc every time they rolled into shop.
There is an order and it is not necessarily IN order I think.
I took a class long ago but forget it honestly, not a lot came thru our place.