What are your ambient temperatures? If it's pretty hot where you are it is possible the TEC cooler can't get low enough to go below 0. I have this problem in the summer here in Vegas, although the Pro model should have a little better range than my older 1600MM-Cool.
Does the sensor show any different temperatures depending on the ambient environment? The 1600 has separate power connectors for the fan and the TEC cooler, some were known to have a loose fit with the TEC resulting in failure to cool and the temp sensor always reading room temp. I had this problem on a 1600MC (which I got for a great price for exactly that reason! 1 minute fix, works great now), if this is your case I can forward you the instructions that ZWO sent me. Super easy.
If the sensor always just reads 0 regardless of ambient temp I don't have an explanation. I'm assuming you've rebooted computer, double checked USB connectors, verified image download / gain and exposure settings work correctly? I think SGP indicates if it can't read the temp from the camera, but I'm not sure what Maxim does if it has a bad connection. If you don't have any luck with the above you could try emailing ZWO directly at email@example.com.