Zeitronix ZT-2 WB controller grounding

I use a Zeitronix ZT-2 WB as both a WB oxygen sensor controller and datalogger to log engine speed and TPS % as it has this capability. AFR reading shouldn't be a concern as it’s send via serial cable to my computer. However, after performing the TPS reset procedure described in the manufacturer documentation to set 0-100% I always have a mínimum reading of 2 to 3%.

The unit gets 12V power supply and power ground as instructed. I think this my be causing my issue being the ground reference different to the TPS sensor ground.

Do you think this may be causing the issue? Would you recommend grounding the unit to sensor ground? I haven’t tried this as I don’t know if it can provoke some kind of side effect to the controller, the ECU or both.

For further information, the ECU is (normally) a Magneti Marelli 5AM which has output for two oxygen sensors, including power to their heating element.