Having problems with your 2004 GMC Sierra ?
2004 gmc 1500 heat is only blowing on passenger side. driver side is blowing cold air.
Sounds like you have dual zone climate control in your truck. Most likely the actuator that controls the temp door in the HVAC box
If the A/C works fine and the issue is only with the heat, check the liquid heat exchangers as the feed from the engine into the A/C area.
Repair Help & Product Troubleshooting for 2004 GMC Sierra
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