bad knock sensor-> limp mode?
 

My PFC hand controller is set to show the Knock on my engine, but it shows blank. So either something is hooked up wrong or the sensor is broke.

Does this make the ECU go into limp mode when the Datalogit is set to use Knock sensor reading? or will it make the car run bad?

So in other words... when Knock sensor reading in Datalogit is set to be 'on', but the sensor is bad , will it make the ECU go into some sort of 'safe mode'

The PFC natively doesn't do anything with the knock reading but display it. I'm not sure if adding the datalogit into the mix changes anything. You'll probably get more help posting in the PFC section:

https://www.rx7club.com/power-fc-forum-47/