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HO2S Aging, Oh, Really?
Posted to Technical Theory Forum on 6/13/2009 7 Replies

Hi, TThF,

HO2S aging DTC is nothing new, neither is rear HO2S' influence on pilot Lambda control. A recent case study made me thinking about those issues.

I have seen a few cases of pre-CAT HO2S aging DTCs. MB's way to detect is to use what's called Tv (not the one you watch at night.:-), it's the dwell on STFT, we have seen how this is at work when it's designed to biased slightly rich or lean. But, MB system doesn't bias that way. It's a Lambda 1.000 control, instead, it measures this Tv to determine if pre-CAT HO2S is aged enough. As those aged ones will cause larger than normal Lambda excursion.

2005 MB ML 500 came in for MIL on, not the 1st time. "George, can you figure out this?", "Let me try" ...; BTW, this vehicle is right before MB started using A/F sensors pre-CAT.

SDS short test


Okay, both banks pre-CAT HO2S are switching. They say rear HO2S can stay anywhere it feels like, right?:-) So lets say it's okay too for now.:-)

I did look at LTFT and STFT, a little strange, but no smoking guns there.


STFT with a little swing

Next, I went to look at Lambda control graphing, basically, HO2S graphing on SDS. Sorry for no color and poor resolution, the best I can show you for SDS image.


2000 rpm

Huh? Interesting. Why B1 pre-cat HO2S say it's lean biased and rear says it's rich? And completely opposite for B2?

I did verify sensor readings are correct comparing on-board and off-board, they matched. SDS guided test also mention to test HO2S heaters, I did that too.

Bx1 Heater Voltage & B1S1 Heater Current

The measurement on peak current to the heater is a little over 500mA, both banks are the same. By now, it's obvious, this is not a large Lambda excursion, but a pre-CAT HO2S and after-CAT HO2S don't agree with each other. It turned out to be a cross connected rear HO2S.

These are after "repair" testing results.

After "repair" Idle

After "repair" 2000 rpm

From this case, what I have realized. 1, Rear HO2S influence on Lambda pilot can be quite strong (I couldn't find any document in detail in MB WIS, I have to piece them together by myself, I wish I didn't have to). 2, What does it mean "delay time is too short" in DTC? I figured, it's negatively too long.:-)

Is there any other auto manufacture(s) also using these?

I didn't measure oxygen content, I wonder what it could be?:-)


George from California

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