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?