GMC Troubleshooting Questions? Ask a Mechanic for Answers
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This can be caused by a restriction in the defrost grille or heater box blocking flow to the passenger side defrost.On some models there are two mode doors for the defrost,one for the drivers side and one for the passenger side.If the passenger side mode actuator is faulty this can also cause this here.I would have the actuators checked/tested to see if any need replaced as well as the defrost grille checked to see if these are o.k.If these are all found to be o.k. then I suspect an internal blockage in the heater box itself with it needing possibly removed and then inspected and repaired.
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Looking at your model here this one actually only uses one mode actuator.It is shown below.
If the air is blowing cold on the passenger side and warm on the drivers when defrost is on then the fault is with the temp actuator for the right side as shown below.If it is blowing warm but low or no air flow from the passenger side of the defrost then this usually indicates a restriction in the passenger side defrost grill or an issue in the heater box itself internally with a door that is stuck not opening properly and the heater box usually needs removed and taken apart for inspection and repair.
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I apologize for the delay.Since it is not blowing this usually indicates a broken door in the heater box or something fell into the defrost vent blocking flow or part of the interior heater box seal came apart in the heater box blocking flow.This would indicate the heater box usually needs taken apart for inspection and repair.
Correct.Since there is only one mode actuator this indicates the mode actuator is working since the drivers side blows.The fault then is blockage or restriction in the heater box that is blocking air flow on the passenger side.The heater box usually needs removed and taken apart and repaired to resolve this here.
Unfortunately on this model the entire dash has to be removed.You might see if you can remove the blower motor and any other bottom components from the heater box and get lucky and find a component jammed blocking air flow at the bottom of the heater box but if this is all found to be o.k. then the dash and heater box has to be removed.