2002 Chevy Blazer Will not change heat direction

Try this:Some customers may comment that the HVAC mode switch may be inoperative or stuck in the vent mode. After some diagnosis, oil in the HVAC control head and/or vacuum lines may be found.

Cause
When the transmission or transfer case heat up, the oil may expand and find its way into the transfer case vacuum switch. This oil expulsion may cause the vacuum switch seal to fail and allow transfer case oil to be drawn into the HVAC vacuum lines.

Correction
Replace the transfer case vacuum switch. Refer to the Transfer Case Vacuum Switch Replacement procedure in the Transfer Case sub-section of the Service Manual. Also, replace all contaminated parts in the HVAC system.

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