Codes P0056, P0141 and P0036

Hello, I'm getting these O2 sensor codes. It used to show P0056 and P0141, Now the P0141 changed to P0036, Interestingly, they (P0141 and P0036) both points towards bank 1 Sensor 2 and these are Heater Circuit Codes, it once found the codes definition to be O2 Sensor Heater Circuit High voltage? I have replaced the O2 Sensors multiple times, with factory and aftermarket sensors, no change. To rule out that its not the O2 sensors itself, I have tested the resistance and continuity, 4.5-5Ohms and passes continuity tests.
Next, I checked the heater circuit itself from the harness pin side to see what kind of voltage I get there. I unplugged the O2 sensor connector,with the car running, I initially tried to use my test light, that didn't light up. I then connected my multi-meter to the heater circuit pins and got 13.5V. Now that's undeniably battery voltage (which explains why the test light didn't light up). So out of curiosity I decided to check if the same voltage is present at the upstream O2 sensors (which by the way does not have any problems) the upstream heater circuit pins has 7.3V which also lights up the test light very dimly.
I believe this is the cause of my O2 Sensor check engine light.

i believe most of the engine electrical's in these cars are controlled/regulated by the PCM.

So, I'm asking what is causing battery voltage to be going to the O2 Sensor Heater Circuits?