Heating problem on 2006 Grand Prix

[Nibcrom]

I haven't had much time to diagnose this problem, but here's what I think I know:

I have a 2006 Pontiac Grand Prix that has a problem with heating up the interior of the car. The heat seems to work fine when you select the upper vents, but if you select the lower vents or the defroster, it blows cold air.

Anyone know what might be causing this?

 

[Fivefingerdeathpunch]

Blend door issue.

Kinda common to have happen. Find the faulty door and replace it or see if the vacuum line came off it.

 

[Nibcrom]

Thanks for the reply, Fivefingerdeathpunch.

I didn't have the time to look at this issue myself, so I took it to the mechanic. It ended up being a blocked heater core and a broken thermostat.