an aged exhaust gas oxygen sensor simulator is inserted between an exhaust gas oxygen sensor and an electronic control module for testing the response of fuel control and emission systems to various amounts of exhaust gas oxygen sensor aging / degrading . the sensor signal is input to a summing amplifier where a delay signal is added . positive and negative summing amplifier outputs go through vernier and decade controls , which add delay , and then to a variable gain inverter . the difference signal is integrated and output to the summing amplifier as the delay signal and output to a ground offset control for shifting the waveform up and down before going out to the control module . the vernier and decade controls add delays to mimic an aged / degraded exhaust gas oxygen sensor .