Oxygen sensor Bank 1 sensor 1 heater circuit P0031

Verified all PCM powers and grounds, cleaned and ran all new body, chassis, and pcm grounds, verified operating voltages at 02 sensor and at the PCM under load. Replaced PCM with new dealer unit and reflashed, installed new oxygen sensors, verified all circuit connections, unplugged everything else that shares the heater ciruit wires and through it all we still keep getting code P0031 reoccuring after only one trip and within sixty seconds even though it is supposed to be a two trip fault code. Voltages and fault code occurence is exactly the same with the new computer and oxygen sensors as with the old parts.