A diagnostic system and a diagnostic method for a thermistor amplifier circuit are provided. The system includes a transistor electrically coupled to a controllable variable resistor having a predetermined resistance. The transistor applies a test voltage signal to the circuit indicative of a first temperature value. An amplitude of the test voltage value is indicative of a simulated thermistor temperature value. The microprocessor determines a test temperature value based on the amplitude of an output voltage of the circuit. The microprocessor also determines an inaccuracy value based on the test temperature value and the simulated thermistor temperature value.