I have a 2004 gmc truck but my heater stops working when the engine rpms runs below 1500 rpms(idaling)the fan works great but blows cold air :eek:

i have a 2004 gmc truck but my heater stops working when the engine rpms runs below 1500 rpms(idaling)the fan works great but blows cold air :eek:
Check for low coolant level and check for the proper concentration mix.
Going on 4 years old, have you replaced the thermostat yet?
Make certain the hoses are getting hot: coolant should be flowing freely.