Exhaust Gas Recirculation (EGR) Modulator The EGR modulator is located on a bracket on the LH side of the engine block, near the starter motor. The EGR modulator is a solenoid operated valve which regulates the vacuum supplied to the EGR valve, causing it to open or close. The ECM uses the EGR modulator to control the amount of exhaust gas being recirculated in order to reduce exhaust emissions and combustion noise. The EGR is enabled when the engine is at normal operating temperature and under cruising conditions. The EGR modulator receives a V supply from the main relay. The ground for the solenoid is via the ECM and is controlled using a PWM signal. The PWM duty signal of the solenoid ground is varied to determine the precise amount of vacuum supplied to the EGR valve, and consequently the volume of exhaust gas delivered to the cylinders. In the event of a failure of the EGR modulator, the EGR function will become inoperative. The ECM can monitor the EGR modulator solenoid for short circuits and store fault codes in the event of failure. The modulator can also be activated for testing using TestBook/T4.