Hello and welcome to JustAnswer. My name is XXXXX XXXXX I am here to help you in any way I can.
The heater blowing cold air can be caused by two different types of problems.
One, you are leaking coolant and the lesser amount of coolant in the system reduces heat output.
Two, the engine is not getting hot enough to generate heat. This means that there is enough coolant in the system, but it is just not getting hot enough.
I recommend you check the following items.
Check for coolant leak. On this vehicle, check the heater core, thermostat housing, coolant bleeder screw, water pump, and rear coolant flange.
If the heater core is leaking, you will see it behind the carpet next to the gas pedal.
If the thermostat is leaking (or the elbow next to it), check for traces of coolant down the driver side of the engine block under the intake manifold.
If the coolant bleeder screw is leaking, you will see coolant around the black screw on the big black pipe on top of the intake manifold.
If the water pump is leaking, look for traces of coolant down the front of the engine, notably on the front of the oil pan.
If the rear coolant flange is leaking, check for coolant leaking at the back of the engine. There should be traces on top of the transmission.
Even if none of these items are leaking, you should be able to find the coolant leak during the process of checking the above mentioned items.
If you don't have a coolant leak (not even a slow leak), then suspect that the engine is not getting hot enough. There are three things that can cause that.
One, the thermostat is stuck open. This causes coolant to immediately cycle through the radiator, which prevents a rise in temperature.
Two, the engine coolant temperature sender is sending the wrong signal to the engine control module, triggering the thermostat to open early.
Three, your fans are stuck on, which causes the coolant to constantly cool off.
All of these items will set a fault in the engine control module. It may or may not actually set a check engine light.
I hope this helps. Feel free to let me know what you find.