Mystery P0155 - tearing my hair out!

[foamypirate]

Hey guys,

Finally at wits end with this one, so thought I'd drop in and see if the pros can help me out here. Just finished up my LQ9/4L80E swap into my 88' squarebody crew, and everything seems to be working great, except for this O2 heater code! It's driving me up a wall. The O2s are setup as case ground, so the heaters are not PCM controlled. I am ONLY getting the code on B2S1. This O2 is actually the closer to the exhaust manifold outlet than B1S1, so it shouldn't be related to placement.

So far I have:

1) Swapped sensors side to side (ACDelco AFS75). Code stayed on B2S1.
2) Replaced the sensor with a Denso. Code stayed on B2S1.
3) Verified 12V on the ignition+ to the heater.
4) Verified resistance on 12v+ to heater is OK (0.5 ohms).
5) Verified resistance on ground to heater is OK (0.3 ohms).
6) Checked resistance on the O2 heater itself (ACDelco was 20 ohm, Denso 7 ohm).
6) Verified continuity between various grounds to the heater ground)
7) Test light from heater 12v+ to frame ground (it lights brightly)
8) Test light from heater 12v+ to heater ground (also lights brightly)
9) For kicks, test light from heater 12v+ to B1S1 heater ground (lights brightly).
10) Used HPTuners to verify PCM is receiving a signal from the O2. It is sluggish compared to B1S1 until driven around and hot, but is receiving a signal.
11) Torn hair out.
12) Thrown things.
13) Pondering setting truck on fire.

The only thing I have not yet done, is verify the resistance on the PCM signal wires, but it seems...unlikely, to my untrained self. I plan to do that today, but wanted to get this out there and see if anyone had any other suggestions before I burn the thing to the ground?

[Oberon67]

You have an '88 squarebody?

[foamypirate]

Originally Posted by Oberon67

You have an '88 squarebody?

Yep, they built the crewcabs, suburbans, and blazers until 91'.

[BR3W CITY]

I don't know "why" its being sluggish, but I'll tell you that the sluggishness is probably whats triggering the code. The ECM is measuring the time switching from open-to-closed loops, so if the singular sensor is slow or way off..it'll trigger the code.

Seems like you've already checked all the old suspects.

[foamypirate]

Originally Posted by BR3W CITY

I don't know "why" its being sluggish, but I'll tell you that the sluggishness is probably whats triggering the code. The ECM is measuring the time switching from open-to-closed loops, so if the singular sensor is slow or way off..it'll trigger the code.

Seems like you've already checked all the old suspects.

I ran a new log today where I let the O2s heat up before firing the engine. It (B2S1) didn't seem as sluggish (in fact, it seemed to heat up faster, possibly due to the Denso sensor having a lower resistance and allowing more current flow).

Here is the HPTuner log, if anyone is interested (right click, save-as). It honestly looks fairly normal to me in most instances, but it is slightly different than B1S1's pattern.

https://foamypirate.com/img/88chevy/...0155_50516.hpl

[BR3W CITY]

Foamy- Firefox has security issues with your certificates and domain reg, so I can't/won't access that log. If you want to email it directly I'll take a look at it. WTFREVJatgmaildotcom

[aknovaman]

Swap out the PCM.

Also, look at this.

http://www.gearchatter.com/viewpost14303.php

[foamypirate]

Originally Posted by BR3W CITY

Foamy- Firefox has security issues with your certificates and domain reg, so I can't/won't access that log. If you want to email it directly I'll take a look at it. WTFREVJatgmaildotcom

Whoops, I accidentally put https:// as the beginning of the URL instead of http://, causing the certificate error (it's not SSL secured).

I'll fix the URL, and shoot you an email with the log as well.

[foamypirate]

Originally Posted by aknovaman

Swap out the PCM.

Also, look at this.

http://www.gearchatter.com/viewpost14303.php

That gearchatter page is great, ended up finding it yesterday searching through ls1tech trying to find some info on my issue.

I have another PCM laying around, so I should be able to try that.
My current PCM is 12586242, and my extra is 12589463. I assume if I replicate the same tune from my current PCM, they are interchangeable? They are both blue/green DBW PCMs.

Here is the correct link to all my logs, by date.

http://foamypirate.com/img/88chevy/l...hp_tuner_logs/

[aknovaman]

One lacks the dbw circuitry. I forget which.

[foamypirate]

Figured it out. In a way, I answered my question with my first post, where I talked about case ground vs isolated ground. In the process, I learned way more about the way O2s work in the LS world than I ever wanted to. In my case, since my PCM is built to switch the heater circuit on via ground (isolated ground O2), and there is no O2 connected to that pin (and thus no current flow through those specific pins), it was triggering the P0155 code. Why just P0155? Because P0135 was turned off in the PCM to be compatible with the case ground O2! Simple case of tuner error, where the P0155 MIL was not turned off.

While I don't find this configuration 100% optimal, since technically there can still be a legitimate P0135/P0155 code with the case ground O2s, it will do for now. All other O2 codes are active, so any more important issues should still light the MIL.