2006 Pontiac Torrent.
Engine Size : 6L
Question : Heater is only blowing cold air wont heat up. The air conditioning works and it switches were it blows the air right.
Answer: OK. You need to find out where the problem is. It could be a problem with the engine having air pockets in the cooling system.
A common cause for that is a bad head gasket. The rear gasket is most common. When it blows, engine cylinder compression is pumped into the cooling system and causes air pockets. This causes a no heat condition. Removing the spark plugs and looking for signs of burning antifreeze is one way to tell. Another is doing a pressure test to look for a leak that may or may not be a head gasket. Any antifreeze leak like a leaking water pump or hose will cause air pockets.
If the engine coolant system is not the problem, then you need to look at the HVAC control inside the truck to see if the temperature door is moving from cold to hot. This SUV along with the Chevy Equinox have a problem where the temperature door breaks in the dash and does not switch from cold to hot. It is located behind the glove box, to the left of it in the HVAC case. You would need to remove the motor that controls the door to check it. If it is broken, the entire dash needs to come out to replace the door. Check this link for some good pictures or temperature door replacement.