Service function guideHeater Not Working Diagnosis

This service function guide walks a working technician through a complete heater performance diagnosis, beginning with how coolant, air doors, and controls interact and ending with a verified repair and a road test that confirms warm airflow at the vents. It is written for mechanics and confident DIY owners who already understand cooling systems and can use a scan tool, a digital multimeter, and a cooling system pressure tester without supervision. By the end you will be able to isolate the fault to the coolant circuit, the blend door or actuator, the blower and mode doors, or the control head, and you will know which measurements prove the cause before any parts are replaced. The procedure also covers hybrid and electric cabin heaters, common diagnostic mistakes, and the aftercare steps that keep a repaired heater working through the season.