An apparatus for testing thermal oxidation stability of a test sample such as a hydrocarbon fuel comprising: a source of electric power; a heater tube connected to said source of electric power for flowing current there through to heat a center section of said heater tube to a predetermined temperature; a coolant flow circuit supplying coolant to each end of said heater tube to keep each end thereof near room temperature; an aeration circuit with an aeration pump for pumping air to aerate said test sample in a sample container; a test sample flow circuit for flowing said test sample around said center section of said heater tube to heat said test sample to said predetermined temperature, said test sample flow circuit including: a single piston, positive displacement, sample drive pump pumping said test sample from said sample container around said center section of said heater tube; a differential pressure filter in said test sample flow circuit after said heater tube to filter out any solids that may have formed in said test sample when heated to said predetermined temperature due to oxidation of said test sample; a differential pressure sensor for measuring differential pressure across said differential pressure filter; a back pressure regulator for maintaining said test sample being pumped by said sample drive pump at a test pressure high enough to keep said test sample in a liquid phase when heated to said predetermined temperature; a waste container for collecting such hydrocarbon fuel after said test; a device for recording said differential pressure and deposits on said center section; characterised in that said single piston sample drive pump is driven by a stepper motor with a motor control, wherein a pulse dampener is located on an outlet side of a pump head of said single piston, positive displacement, sample drive pump and wherein a high pressure, low flow rate restriction coil is located downstream of said pulse dampener such that output flow from said single piston, positive displacement, sample drive pump is dampened. a source of electric power; a heater tube connected to said source of electric power for flowing current there through to heat a center section of said heater tube to a predetermined temperature; a coolant flow circuit supplying coolant to each end of said heater tube to keep each end thereof near room temperature; an aeration circuit with an aeration pump for pumping air to aerate said test sample in a sample container; a test sample flow circuit for flowing said test sample around said center section of said heater tube to heat said test sample to said predetermined temperature, said test sample flow circuit including: a single piston, positive displacement, sample drive pump pumping said test sample from said sample container around said center section of said heater tube; a differential pressure filter in said test sample flow circuit after said heater tube to filter out any solids that may have formed in said test sample when heated to said predetermined temperature due to oxidation of said test sample; a differential pressure sensor for measuring differential pressure across said differential pressure filter; a back pressure regulator for maintaining said test sample being pumped by said sample drive pump at a test pressure high enough to keep said test sample in a liquid phase when heated to said predetermined temperature; a waste container for collecting such hydrocarbon fuel after said test; a single piston, positive displacement, sample drive pump pumping said test sample from said sample container around said center section of said heater tube; a differential pressure filter in said test sample flow circuit after said heater tube to filter out any solids that may have formed in said test sample when heated to said predetermined temperature due to oxidation of said test sample; a differential pressure sensor for measuring differential pressure across said differential pressure filter; a back pressure regulator for maintaining said test sample being pumped by said sample drive pump at a test pressure high enough to keep said test sample in a liquid phase when heated to said predetermined temperature; a waste container for collecting such hydrocarbon fuel after said test; a device for recording said differential pressure and deposits on said center section; characterised in that said single piston sample drive pump is driven by a stepper motor with a motor control, wherein a pulse dampener is located on an outlet side of a pump head of said single piston, positive displacement, sample drive pump and wherein a high pressure, low flow rate restriction coil is located downstream of said pulse dampener such that output flow from said single piston, positive displacement, sample drive pump is dampened.