This paper describes the application of the Fourier Amplitude Sensitivity Test (FAST) method  to investigate the effect of uncertainty in design parameters on the thermal system performance of vehicle underbody components. The results from this study will pinpoint the design parameters which offer the greatest opportunity for improvement of thermal system performance and reliability. In turn, this method can save engineering time and resources.An analytical model was developed for a vehicle underbody system consisting of a muffler, heat shield, and spare tire tub. The output from this model was defined as the temperature of the spare tire tub. The majority of the input parameters in this model deviate from their nominal values due to environmental factors, wear and ageing, and/or variation in the manufacturing process. Using MATLAB software package, the model was simulated with input parameters which were simultaneously and sinusoidally varied at distinct frequencies over their respective uncertainty ranges.Finally, the Fourier transform was applied to the output of the model to convert the response into the frequency domain to allow the amplitude of each of the distinct frequencies to be recognized. The amplitude of each frequency was used as an indication of the effect of the corresponding parameter's variation on the temperature of the spare tire tub. Reducing the amount of variation in the most influential parameters and/or reducing the system sensitivity to these parameters will have the greatest improvement on the overall thermal performance of the system.