Well, the ambient temp thermistor will have no direct effect on the cook, but having it on will put a line on the graph way down low and make the scale of the graph be larger (from room temp to a bit above the setpoint). If you are using a food probe in a meat you are going to have that range anyways, but if you are cooking say ribs or steak and have no probes in food, and don't have your ambient temp thermistor enabled, then your HM graph will display a much more narrow temp range centered around the setpoint instead of extending all the way down to room temp..... but like I said, you can disable the ambient temp probe in software if you like so it's not an issue. In the beginning I always had an ambient temp reading on the screen, then when I started to microscope the performance of the probes and TC and CAT5 cable etc I started to disable the ambient temp. When I had the ambient temp enabled and thermistor installed inside the HM case it always read quite a bit higher than the actual ambient temp, so I've kinda quit using them these days....