Heater Core? Thermostat?

[JDM]

Car only heats when I'm on the throttle. At idle at stoplights and such, no heat.

What is this, and will they warranty it?

[1ViciousGSX]

Thermostat stuck open or possibly low on coolant.

[LightningGSX]

So at idle it blows cold air? or does it just stop blowing air period at idle?

[JDM]

It doesn't stop blowing, just blows cold air at idle.