The heater core is stored within a case attached to the Sierra’s chassis You must disconnect the case assembly, if the core needs replacing. The case has multiple connectors and hoses you’ll have to disconnect. Shutting off the power and draining the cooling system are required, as well.
1. Drain the engine coolant from the cooling system. Use a clean container, if you believe you can use the coolant again, although replacing it with fresh coolant may be best. Crawl under the GMC Sierra, remove the drain plug from the radiator, let the coolant flow into your container and then replace the plug.
2. Raise the hood and take off the Sierra’s negative battery cable. Disconnect the heater hoses from the core. Remove the temperature control cable and mode control cable from the heater case assembly.
3. Disconnect the instrument panel carrier that provides access to the heater case assembly. Undo all electrical connectors that could interfere with removing the case assembly.
4. Remove the case assembly by disconnecting the assembly-to-chassis screws/nuts. Extract the heater core from the case by removing the core cover screws.
5. Install the replacement heater core to the heater case and reconnect the case to the chassis, tightening all screws.
6. Reconnect all electrical connectors that you detached. Reinstall the instrument panel carrier, mode and temperature control cables to the case assembly and the heater hoses to the core.
7. Put the negative cable back on the battery and refill the engine cooling system. Let the engine run to normal operating temperature and check for leaks in the system.