Rke brand new. With the black probe. If you go to the calibrate screen and pull the sensor out of the water the numbers that fluctuate at the bottom move so I know it works. I set it at the temp my water is using a basic thermometer and I click save. So let's say I save it at 77.8 as to what my water is at that moment. It doesn't ever change on my display it stays like that the entire time it doesn't move and I highly doubt my tank is staying 77.8 on the spot for 2 days..