Abstract: The Unused Fuel Indicator (UFI) comprises a reliable thermo-electric/electronic circuit employing thermocouples attached to the catalytic converter of an emission control system for an automobile engine for providing easy readout of the condition of the automobile engine and the emission control system. A preferred kind of readout is provided by the sequential glowing of light indicators such as light emitting diodes (LEDs) wherein the sequence in which the LEDs are caused to glow, and their color is indicative of the proper functioning (or malfunctioning) of the automobile engine and catalytic converter emission control system. The UFI incrementally indicates the potential heat of pollutants in an automobile engine exhaust, which pollutants are converted by oxidation to heat produced by the catalytic converter. The heat is sensed by thermocouples which are differentially connected with the output of one opposing the output of the other.