Why is my heat control unit blinking on your 91 GMC 1500?

A blinking heat control unit on your 1991 GMC 1500 typically indicates a problem with the heating or air conditioning system, such as a malfunctioning sensor or a fault in the climate control module. It could also signal an issue with the blend door, which regulates airflow. Checking for error codes using an OBD-I scanner can help pinpoint the issue. If you're unable to identify the problem, consulting a professional mechanic is advisable.