i have my ranco set slightly higher than my desired temp. It is set at 94 degrees but it holds at 89. I have it set to a low differential. Do you have the probe set on your heat tape, And taped down so that its right against the heat source. That could be your cause for an inaccurate temp aswell. Could you describe your set up so we can figure out if its just placement or if you need to adjust for probe differential