The nozzle consistently fails the G29 autolevel command. My single extruder works just fine. The M175 prints fine if I comment out the G29 command in the M175 machine settings.
On my TAZ 6, my M175 autolevels successfully.
Since your Single Extruder works, I would check that the ground wire is firmly attached to the M175 heater block and then check continuity between there and the connector.
From the connector to the controller board has been already been verified by the Single Extruder. It also verifies that the corner washers are properly connected as well.