I wonder what would happen if you insullated the t'stat housing and sensor...

I would also log the voltage to the ohtwo sensor heaters. I fouund the following:

"P0125 set if after the engine is warmed up, A/F Sensor Output* does not change when
conditions (a), (b), (c), and (d) continue for at least 1.5 min.:
a. Engine speed: 1,500 rpm or more
b. Vehicle speed: 25 û 62 mph (40 û100 km/h)
c. Throttle valve is not fully closed
d. 140 sec. or more after starting engine
*Output values changes inside ECM only


After checking around further, those are the conditions that factor into the fault. So the code haas nothing to do with the coolant temperature sensor gating out the closed loop operation, but instead is related to the O2 sensor readings..

I'll check into the O2 sensor next."