MK7 Rear O2 Sensor: More than a Catalyst efficiency monitor?

[Cliff p.]

Having some discussion in another thread here, the rear o2 sensor "defouler" was brought up. Some mentioned that the rear o2 sensor on the MK7 is more than just an efficiency monitor/sniffer. Obviously, the rear o2 isn't a 5 wire (wideband/lambda) sensor, so this confuses me.



Is there a way the ECM interprets the rear o2 sensor working with the front o2 sensor, such that it will adjust the AFR target? I know that on older vehicles (OBDII without a 5 wire upstream sensor), the upstream and downstream sensors do work in tandem to target AFR set by ECM. I just can't see this solution for cars equipped with a wideband o2 sensor that is obviously more accurate in measuring precise AFR.



Mainly looking for those tuner guys in the audience to chime in, for my personal edification and for that of the community. I think it matters quite a bit if the rumors are true and the rear o2 performs more than a sanity check, and can actually affect AFR targets.

 

[Diggs24]

Those people are wrong. It is nothing more than a catalyst efficiency monitor as you have guessed.

 

[Cliff p.]

Thanks. I wasn't sure if there was some form of wizardry going on or something.

 

[Hoon]

Those people are wrong. It is nothing more than a catalyst efficiency monitor as you have guessed.

I'm waiting for more info on this.

One pro-tuner said it's used for fueling and another said it's just an efficiency monitor but if you trip it, it will affect target AFR.

 

[lou06483]

FWIW--The rear sensors, besides being used to check for the efficiency of the converter, are also used as the final say for fuel trims--Bosch has been doing this since their ME7 ECU's---Yep- that's right---the second sensor can over-ride what the first sensor "says". I'm STILL looking for the white paper on this for our ECU---(got plenty of stuff on the ME7)....

 

[Joel]

My Stratified tuner said if the spacer is giving you no CEL to just leave it alone...they quit not long after, been rocking it nothing turned off and the spacer that gives me no CEL. would like to know if I am in error.

 

[Diggs24]

https://www.golfmk7.com/forums/showpost.php?p=282572&postcount=6

 

[Hoon]

FWIW--The rear sensors, besides being used to check for the efficiency of the converter, are also used as the final say for fuel trims--Bosch has been doing this since their ME7 ECU's---Yep- that's right---the second sensor can over-ride what the first sensor "says". I'm STILL looking for the white paper on this for our ECU---(got plenty of stuff on the ME7)....

Good stuff, thanks.

Both of the tuners I've talked to say there is an effect from the secondary, but they don't agree on the scope of the effect.

One says it's used in fueling calculations, the other wants to do more research but believes it only affects target AFR after it detects catalytic inefficiency.

 

[Ed @ EQT]

If anything, the rear O2 could affect AFR targets near stoich (at cruise conditions). Since its a narrowband, I don't see how it could have any bearing on AFR targets richer than around 13:1.

-- Ed

 

[uberpolka]

le sigh


Guess I'll follow this thread for a bit :-/

 

[Wrath And Tears]

The rear 02 has been used for more than cat health since the 90s (I don't want to call it fuel control, but it can influence it). VW used it in the past, no reason that they have stopped now. Should be pretty easy to find out by looking in VCDS.

https://mechanics.stackexchange.com...oxygen-sensor-used-for-engine-mixture-control

Good stuff, thanks.

Both of the tuners I've talked to say there is an effect from the secondary, but they don't agree on the scope of the effect.

One says it's used in fueling calculations, the other wants to do more research but believes it only affects target AFR after it detects catalytic inefficiency.

I know that BMW uses it in both functions.

 

[cr_sti]

I know that BMW uses it in both functions.

Subaru also uses the secondary O2 for fuel trims, but only during lean burn/cruise conditions.

On that platform removing the secondary O2 made no significant impact, especially as it relates to power/reliability. I expect that's true on the MQB as well, but would be interested to hear more.

 

[949]

so will having an aftermarket down pipe with a high flow cat cause the car to have major issues and run rich/lean?

 

[Cliff p.]

so will having an aftermarket down pipe with a high flow cat cause the car to have major issues and run rich/lean?

No. That’s kind of the point to this thread. Some say that yes it absolutely matters because the rear o2 works with the front o2 to control AFR targets. These cars have wideband o2 sensors for primary sensors, and thus report to the ECM for AFR targeting.

Now I can understand trims being slightly affected by the rear o2 sensor during idle/cruise conditions, but never so much that you’d be adversely affecting the tune of the car like some have said.

For reference I run a catless with defouler and don’t notice anything odd. That’s why I wanted to learn more about what some have claimed. If my targets can in fact be affected and cause issues with fueling, then I would want it rectified.


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[tristanr]

old thread, but the cat inefficiency code with my APR downpipe only triggers after cruising on the highway.