Miata Turbo Forum - Boost cars, acquire cats.

I'm running a maxxecu. I am using diyautotune's N122 breakout board as a plug and play adapter... My symtomp that I noticed is, the Miata NB2 coolant sensor isn't reading correctly. It will start at the lowest temperature set in the tune and then rapidly increase reaching the max set temperature in the ecu within 1-2 minutes. When the car is in accessory ON but engine off, the temperature on the sensor remains as is OR when I shut the car off, the number will gradually drop assuming the car was running seconds earlier... but if I leave the car running for awhile, it takes a good amount of time to lower on its own as it'll hang at it's maximum value for awhile before dropping. It's like I have a chassis and sensor ground mixed somewhere and as noise across the chassis ground increases, it affects the sensor ground making the resistance(?) seem higher leading to my bad temperature reading.

I quadrupled check, and I ONLY have the chassis pin on the maxxecu connected to Pin 4A on the miata ecu header.
I only have the maxxecu sensor ground connected to 4O on the miata header.

Any idea on how to troubleshoot this? How can I test my dirty sensor ground hypothesis? I know all the wiring on the miata side is perfect (engine harness) so the fault lies with my ecu wiring.