The deflection signal of a thermally excited force sensor of an atomic force microscope can be analyzed to gain important information about the detector noise and about the validity of the equipartion theorem of thermodynamics. Here, we measured the temperature dependence of the thermal amplitude of a tuning fork and compared it to the expected values based on the equipartition theorem. In doing so, we prove the validity of these assumptions in the temperature range from 140 K to 300 K. Furthermore, the application of the equipartition theorem to quartz tuning forks at liquid helium temperatures is discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3627184]