The intake heater in a Duramax diesel engine is designed to warm the incoming intake air in order to create effective cylinder combustion. The engine control module, or ECM, decides when to turn on the intake air heater. When the ECM turns on the heater, it grounds the middle coil on the intake air heater relay, supplying power to the heating element. Replacement intake air heater relays are available from licensed General Motors dealerships.
Park the vehicle on a flat surface and shut down the engine. Set the parking brake and open the hood. Let the engine cool down for at least one hour.
Locate the intercooler. It is in front of the radiator, and looks like a second radiator. It is made from aluminum and contains a large number of cooling fins.
Follow the intake hose on the left side of the intercooler to the intake manifold where it terminates. The intake heater is positioned inside this line, approximately one inch from the intake manifold. A small black plastic box attaches to the intake hose with a wire that protrudes from the back of the intake heater. The intake heater relay is inside this box. The cover on the box snaps off to reveal the relay.
