Mystery P0155 - tearing my hair out!

[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.