NONCOMBUSTIBLE. A material that in the form in which it is used and under the conditions anticipated will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat, as defined by fire protection industry standards on the basis of large scale fire tests performed by a nationally recognized independent fire test authority. SOURCE: Department Of Energy
Diesel Fuel is considered to be noncombustible.
A DEF test will help determine the quality of the fuel if it has contaminants or not.