Plugged heater core?

[Bluejediz]

I believe all GM products use hot bottles, meaning the "overflow" tank is pressurized with the coolant system. Our cars use regular overflow tanks that are AFTER the radiator cap, meaning they are not pressurized with the coolant system.

If this is true and your "hot bottle" was low, then your system really was low on coolant.

Edit: Has your heat worked properly since your coolant flush?

Some GM's use the "hot-bottle" system, some dont. The 07 GP's dont, and the overflow tank is only suppose to have a very small amount of coolant in them so they get low easil but usualy dont cause problems when low.
Sounds like RealMcCoy is on the right track....
Make sure your radiator is full.
Get the car nice and hot and turn the heat all the way up. Grab both heater hoses at the same time and if your heater core is working correctly one will be alot cooler than the other.
If its plugged they will both be hot. So then take both hoses off and flush it back and forth. Youll be amazed with the crap that will come out. Especially with the GM's DEXCOOL...that shit gets SO nasty.

This is where I would start since its relatively easy and free and regardless if its the problem or not its still a good thing to do for maintanence.

[Alex3000gt]

Some GM's use the "hot-bottle" system, some dont. The 07 GP's dont, and the overflow tank is only suppose to have a very small amount of coolant in them so they get low easil but usualy dont cause problems when low.
Sounds like RealMcCoy is on the right track....
Make sure your radiator is full.
Get the car nice and hot and turn the heat all the way up. Grab both heater hoses at the same time and if your heater core is working correctly one will be alot cooler than the other.
If its plugged they will both be hot. So then take both hoses off and flush it back and forth. Youll be amazed with the crap that will come out. Especially with the GM's DEXCOOL...that shit gets SO nasty.

This is where I would start since its relatively easy and free and regardless if its the problem or not its still a good thing to do for maintanence.

Ah, good to know.

On my 3000GT I filled up the heater core with CLR for five minutes then flushed it out both ways with a hose. Still had no heat. That car drove me nuts, I'm so glad I parted it out!

[j2k4]

Might be a wonky heater control valve.