Instructions to Repair a Heater Core in an Envoy

by Melissa Kelly
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A View of an Air Vent on a Car. image by daseaford from

The heater core on a GMC Envoy is located behind the instrument panel and dashboard. A failed core can lead to windows fogging up or coolant leaking from under the dashboard onto the passenger side floor board. The replacement process is the same for all models, independent of the trim package or engine type. It can be rather time consuming since many components must be removed.

Step 1

Discharge the air conditioning system to relieve the pressure and dispose of freon properly. It is best to take the vehicle to a service center or dealership to ensure it is discharged according to EPA guidelines.

Step 2

Remove the cable connected to the negative terminal of the battery.

Step 3

Drain the coolant from the radiator by placing a container below the radiator drain and loosening the hose clamp on the lower radiator hose. Slide the hose back until the coolant begins to drain and then remove the radiator cap.

Step 4

Remove the heater hoses and air conditioning evaporator lines from the fittings at the firewall. Cap the fittings with tape to prevent coolant spillage and prevent moisture from entering the system. Heater hoses may be cut off if you are unable to remove them easily.

Step 5

Remove the instrument panel assembly by first removing the upper trim pad. Remove the trip strips. Carefully pull up the trim clips along the upper trim pad. Twist the ambient light sensor one-quarter turn counterclockwise and remove it from the trim pad. Remove the screws and remove the center console, the sound insulator panels (left, right and center) and the knee bolster with the trim tool. Remove the left accessory trim panel, the center accessory trim panel and the instrument cluster bezel. Remove the glove box.

Step 6

Remove the instrument panel carrier and heater/air conditioning assembly (HVAC module) assembly. Separate the HVAC module from the instrument panel carrier.

Step 7

Remove the screws and separate the cover from the HVAC housing. Remove the screw and clamp that retain the heater core. Remove the heater core.

Step 8

Reinstall the new heater core into the HVAC housing. Make sure the original insulation/sealing materials are in place around the heater core pipes and core.

Step 9

Reinstall the remaining components in the reverse order of removal.

Step 10

Reconnect the lower radiator hoses to the radiator and install the clamp. Slowly fill the radiator with coolant until it's full.

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