Circuit/System Description
WARNING: This page is about a different car, the 2019 GMC Savana 3500, 2019 GMC Savana 2500, 2019 Chevrolet Express 3500, and 2019 Chevrolet Express 2500. However, it is still accessible from the selected car via links, so may be relevant.
Ignition voltage is supplied to the coil side of the fuel heater relay whenever the ignition is ON. The Engine Control Module (ECM) provides a ground to control the fuel heater relay. When the ECM is commanding the fuel heater On, the voltage of the control circuit should be low, near 0 V. When the ECM is commanding the fuel heater Off, the voltage of the control circuit should be high, near battery voltage.