Just wondering if the downstream 02 is there just for emission purposes or does it actually affect the fuel trim.

 

Pretty sure the primary O2 is for fuel trim and the secondary is for monitoring the efficiency of the catalytic converter. O2 is basically a thermal sensor. Elevated exhaust tenon indicates a lean condition cooler temp indicates a rich condition.

 

^ +1

 

Yes, it does.

 

 

I understand the primary is the only one that monitors fuel mixture, and the secondary only monitors cat efficiency.

 

So which one is it ?? I changed my primary 02 cause i had a check engine light for it.. When i changed it out the check engine light went off for a while but retuned back, for the same bank but downstream... And while it was on, the car didnt feel as peppy for sure! Thats the reason i asked actually. The light went off and the car is back to normal, which makes me think that it does effect it. I do have HFC but didnt have a cel from the beginning and for a long time.

Im thinking to change the downstream ones too.

 

Upstream o2 is used for determining fuel trim. It does not care if you have a cat or not.
Downstream o2's main job is to monitor cat efficiency. It also has an impact on fuel trim but very minimal compared to upstream ones.

 

Never looked at the TL specifically. Most of the time the primary sensor is the only one that controls fuel trim. The secondary is mostly looking for a change from what the primary sees. If the secondary has any influence its likely in the interest of emissions and converter life, not necessarily what's best for the engine. People fool the secondaries all the time or use resistors in their place with no detrimental effects so of it has any influence in the TL its very minimal.

Never will the secondary O2 have even close to the influence of the primary. Primary is a wideband meaning just what it sounds like, it's accurate over a wide range of mixtures. The "regular" O2 is only accurate near stoich. The farther you go from the "center" which is usually stoich, the more inaccurate they get. In the old days you used to look for crosscounts per second, crossing over stoich on both sides for best catalyst efficiency and the O2 was not used for full throttle because it was too inaccurate for the richer full throttle conditions.

With that said I don't know the answer for sure but if the secondary is used at all its very little.

 

From the FSM:
Secondary Heated Oxygen Sensor:
The secondary HO2S detects the oxygen content in the exhaust gas downstream of the warm up three way ctalytic converter, and sends signals to the ECM/PCM. To stabilize its output, the sensor has an internal heater. The ECM/PCM compares the HO2S output with the A/F sensor output to determine catalyst efficiency.

Just an FYI-The A/F sensor is the upstream HO2 sensor.

 

That really makes it sound like the secondary is just for checking converter efficiency especially the terminology used.

 

Just like IHC said...
and exactly what I heard from dad's mechanic friend

Secondary is just checking the primary one blah blah, top one is the one you would want to replace in case of MPG drop

which I need to as well

 

Don't get too excited:
Won't post the testing and results, as it is a long process, but here's the condensed version.

Second O2 sensor and STFT (long)
What is the purpose of the post cat Oxygen sensor?

Catalyst efficiency monitor?

That's only a portion of its function

REAR FUEL TRIM

a.. The Post-cat O2S has been used since 1988 to control fuel trim (to
some extent)
a.. Toyota and Saab
b.. Used to fine-tune the A/F ratio to maximize catalyst efficiency
c.. Will also compensate for a degraded Catalyst
d.. Every manufacture today uses the rear O2S to trim fuel today


The post cat O2S is a fuel control input. Toyota and Saab have used the
post-cat O2 to control the fuel trim since 1988; Before OBD-II. This sensor is used to fine-tune the air-fuel ratio to maximize catalyst efficiency. It can also adjust the air fuel ratio to compensate for a degraded Catalyst. If you were to measure the actual amount of time the post-cat O2 is used during the running of the catalyst monitor (maybe once per trip) and compare that to the amount of time the post-cat O2 is used fine tune the air-fuel ratio (almost always in closed loop), you will see that this sensors major function is fuel control. Every vehicle manufacture today uses the rear O2S for fuel correction; even if it is undocumented.

a.. How much control over fuel trim does the rear O2S have?
a.. GM material says less than 1.0%
b.. Ford says 0.5%
c.. Toyota says 2.0%
d.. Under normal conditions. We have seen up to 30% correction in fuel trim from the rear O2S in abnormal conditions

 

I know a guy that had a 99 Civic with catalyst efficiency issues. He had the downstream O2 free air at one point. The car was unaffected when cold, but after a few minutes it would barely run and was choking on gas.

 

That's pretty screwed up if that's the case because it allows an old or non functioning catalytic converter to screw with what's optimal for the engine. 1%, fine, but anymore and you will see it at the pump. 30% is rediculous and that engineer should be fired but I'm sure it's because of government pressure to put emissions first, before all else.

I also think the word "fine tuning" the AF ratio is the wrong way to describe what it does. It does not fine tune for the engine's needs, it fine tunes for what's best for catalyst efficiency and an educated guess is however much the secondary is "compensating" your AF will be that far off from what the engine actually needs. The primary actually gets the AF right in the first place, the secondary screws it up when it compensates.

Good info, where did you get that from, I would like to see if it mentions Honda anywhere.

 

Matt, no mention of Honda, but I'm with you in that I don't know anyone that has "altered" the secondary O2 sensor and have run into a problem, but that may be because of the low percentage of secondary value.

http://forum.aempower.com/forum/index.php?topic=23625.0