GM: Eliminating Air Conditioning Odor – UnderhoodService

GM: Eliminating Air Conditioning Odor

GM Air Conditioning odorModels: 2014 and prior GM passenger cars and trucks

Some customers may comment about musty odors emitted from the HVAC system at startup in hot, humid conditions.


This condition may be caused by condensate buildup on the evaporator core, which doesn’t evaporate by itself in high humidity conditions. The odor may be the result of microbial growth on the evaporator core. When the blower motor fan is turned on, the microbial growth may release an unpleasant musty odor into the passenger ­compartment.

There are several other possible sources of a musty odor in a vehicle. A common source is a water leak into the interior of the vehicle or foreign material in the HVAC air distribution system. Follow the procedures in the service information (SI) for identifying and correcting water leaks and air inlet inspection.

This procedure is only applicable if the odor source has been determined to be microbial growth on the evaporator core inside the HVAC module.


Many vehicles currently incorporate an afterblow function within the HVAC control module software. The afterblow feature, when enabled, employs the HVAC blower fan to dry the evaporator after vehicle shutdown, and this function will inhibit microbial growth. Technicians are to confirm that the customer concern is evaporator core odor and that the vehicle has the imbedded afterblow feature, as defined in the SI document for that specific vehicle model, model year and specific HVAC option. Refer to SI for enabling the afterblow function. Vehicles in areas prone to high humidity conditions may benefit from having the afterblow-enabled calibration installed prior to any customer comment.

To immediately remove the evaporator core odor on all suspect vehicles, it is necessary to eliminate the microbial growth and prevent its ­reoccurrence. To accomplish this, perform the following procedure:

1. The evaporator core must be dry. This may be accomplished by disabling the compressor and running the blower fan on the recirc heat setting for an extended period of time.

Note: Compressor engagement will cause the evaporator core to remain wet and will prevent full adherence of the cooling coil coating to the evaporator core surfaces.

2. Verify that the air conditioning drain hose is not clogged and place a drain pan beneath the vehicle.

3. Place a protective cover over the carpet below the evaporator core.

4. Remove the cabin air filter, if equipped, and cover the opening prior to applying a cooling coil coating, as the product may clog the filter. If the cabin air filter appears to have little or no remaining life, suggest a replacement to your customer.

5. If the HVAC module has a blower motor cooling tube, be careful not to spray the cooling coil coating into the blower motor cooling tube.

6. Attach a flexible applicator pressure spray tool to a compressed air line operating at 85 psi to 115 psi.

7. Shake the bottle of cooling coil coating well. Screw the bottle onto the cap on the applicator tool’s pick-up tube.

8. To apply the coating, follow these steps:

• Go through either the blower motor control module opening or the blower motor opening.
• Remove the blower motor control module (blower motor resistor). Refer to the applicable procedure in SI.
• Clean any debris or foreign material from inside the HVAC module and on the evaporator core surface.
• Apply the cooling coil coating directly to the evaporator core through the blower motor control module (blower motor resistor) opening.
• Use the flexible wand to direct the cooling coil coating over the entire evaporator core and surrounding gasket surfaces.
• When the application is complete, install the blower motor control module (blower motor resistor).
Note: If neither of the blower motor control module opening or the blower motor opening are available, it may be necessary to drill a hole in the HVAC module:
• Locate an area of the HVAC module between the blower motor and the evaporator core. Drill a 10 mm (3/8 in) hole in the HVAC module. Use caution to keep the drill clear of the evaporator core and the blower motor fan.
• With the air distribution vents closed and the blower motor fan speed on high, insert the applicator tool into the hole and spray the cooling coil coating into the airstream toward the evaporator core.
• Use RTV sealant to plug the hole in the HVAC module.

9. After the coating application is complete, start and run the vehicle for approximately 10 minutes, with the compressor disabled, HVAC mode set to recirculate/max, heat set to full warm, blower motor fan speed on high, and one window open approximately 12 mm (1/2 in). This cures the coating onto the evaporator core surface.

10. Shut off the engine and enable the compressor again.

11. Verify proper HVAC system operation.

12. Remove the protective cover from inside the vehicle.

13. Remove the drain pan from underneath the vehicle.

14. Reinstall the cabin air filter if necessary.

Courtesy of ALLDATA

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