Passenger vehicles like buses tend to soak up heat when they are parked under an open sky. The temperatures inside the vehicle can get very high during daytime due to heating, which reduces the thermal comfort levels. All three modes of heat transfer, i.e., conduction, convection and radiation contribute to the heating process. Cool-down tests are performed to replicate this thermal behaviour and evaluate the time required for cooling the internal bus volume to comfortable temperatures. The phenomenon can also be analysed using CFD, and accounts of numerous such studies are available however, the effects of all three modes of heat transfer for practical application are rarely studied. In view of this, an effort has been made to develop a fast and reasonably accurate transient numerical method to predict the thermal behaviour of the cool-down process for a school bus cabin. The effects of all three modes of heating (conduction, convection, and solar radiation) have been evaluated, and the method has been validated using test data. Comparison of CFD & test data shows that CFD tends to predict faster cooling, which can be attributed to lesser thermal mass inside the bus due to the simplification of the bus model for practical purposes of reducing the simulation time. The overall deviation of the CFD results from the test data is in the order of 3-8%, which was found to be reasonable given the computational time, however, the accuracy of the method can be further improved by evaluating different turbulence models. The method can be used for comparing the cooling performance of various evaporators and evaluate passenger thermal comfort inside the bus cabin, ultimately help in reducing the energy consumption of the vehicle.