sockeye

Hello
My issue is a persistent code 21 (O2 sensor heater circuit) following the harness melted against the exhaust. After cutting out approximately 3 inches of melted wires and soldering the harness together, the code 21 remained after an ECM reset. After reading threads regarding the problem I pulled the ECm and found a burnt resistor where other have had, connected to HT1. Replaced it with a 0.3 Ohm resistor which restored continuity to the O2 sensor heater. Checked the resistance across the heating circuit on the oxygen sensor itself and is was below spec at 3.8 Ohms, and replaced it with a Denso unit that tested at 5.8 Ohms. When I turn on the ignition, engine off, I measure ~12 volt across the heating circuit wires at the sesnor, and I can feel the sensor warming up. However I still have a code 21 after ECM resets that comes on immediately with igniton.

Multimeter readings:

Pink/Green to Red/White (at sensor)= fluctuates around ~12V
Brown[sensor ground] to Body Ground = 0.5 ohm
HT1 to Body Ground (at ECM w/ IGN off) = 75 ohms
(w/ IGN on) = 420 ohm
OX1 to Body Ground (IGN off/ Cold motor) = 3.6 mV
E1 to Body Ground (Ign off) = 2.5 ohms
(Ign on) = 3.5-6.0 ohms

I was also getting a steady 0.7 V across VF1 and E1 (w/ E1 and TE1 jumped), but I have a modestly priced multimeter without graphs. At this point I think my repair on the ECM works enough to heat the sensor but not well enough to satisfy the CEL. I've read just about every O2 sensor thread I can find and I have the FSM, but on the flow chart I'm caught about where the voltage should fluctuate but mine does not, that steady 0.7 V. Any suggestions before I purchase I go and buy a new-ish ECM would be much appreciated, thank you.