66% of those fires occurred in standard passenger vehicles. Causes of vehicle fires unintentional actions accounted for 38 percent of highway vehicle fires. These fires may be the result of either careless behavior or accidental actions. Fires caused by intentional actions constituted only 5 percent of highway vehicle fires. The NFIRS no longer collects information on suspicious fires that may have been intentionally set.  As a result, fires caused by intentional actions may be undercounted. Moreover, due to the volume of fires, many automobile fires are not investigated for possible arson. Determining the cause of vehicle fires is often challenging. As a result, in 23 percent of highway vehicle fires, no cause was determined after an investigation. The failure of equipment or the heat source was the cause in an additional 21 percent of highway vehicle fires. These fires are generally a result of mechanical problems, ranging from a faulty design in the vehicle to an improperly installed device.
source: USFA