A thermal oxidation tester is shown for determining thermal stability of a fluid, particularly hydrocarbons when subjected to elevated temperatures. The tendency of the heated fluid to oxidize and (1) form deposits on a surface of a heater tube and (2) form solids therein, are both measured at a given flow rate, temperature and time. The measured results are used to determine whether a fluid sample passes or fails the test. Results of measurements can be recorded in a memory device on one end of the heater tube on which the deposits were made. A method and apparatus is also provided to determine if the isolated heater tube has an electrical short to ground through the test section housing.