2 yrs.,ago, i had to replace the water pump for my 2006 pontiac grand prix as well as the thermostat, that was in october, when winter came in late january of the following years, no heat. verything

I believe u are looking in the right area. How many miles on your car? When u can tell me the color of the exhaust when idling? Is it white? Have you replaced the radiator cap in a while as well? What does your temperature gauge read along the way?

Also check...if both hose to the heater core are hot and the heater core is not leaking there is probably nothing wrong with the heater core . I would check to see if the diverter door is working . it usually works on vacuum and changes the direction of the air flow from vent to heat or ac when called for

I had a similar issue to this on my 2006 Grand Prix. After painstakingly removing the glove box to look at the blend door actuator, I found it to be working normally. In hindsight, I didn't need to do all of that work, because I could actually hear the actuator working when I changed the temp control knob. You should be able to hear it run and bottom out when you are all the way cold or all the way hot. This shows that both the actuator is working and the blend door is not jammed. I suggest the next step will be inspecting the heater control valve, and I am confident that this will fix it.

Lastly, this is a very common problem in Pontiacs. I have changed the water pump, thermostat, and radiator. Most likely what happen is someone mixed 2 different kinds of coolant and it created a orange clay that clogs the radiator and the heater core. If u have this orange clay in your car, u will go threw hell to get this fixed. First off u need a new radiator, cause there is no way u r going to get it clean, I tried. When u install the radiator do not connect it to your cooling system yet. Next, flush the inside of the engine with hot water, when flushing, take a break and let hot water sit in the engine to help break down the orange clay, and keep flushing out. This will take awhile. Then splice the heater core lines, and get connectors to put the heater core back together. Now I got muriatic acid and I poured about 3/4 of a cup down in the heater core and had pressured hot water ready to shoot threw the line after u pour the acid in. Do not let that acid sit in there, if u leave the acid in too long it will ruin the heater core. After u flush the acid out, have a air hose to shoot threw the line and shoot all the water out. Keep putting hot water in the heater core and blowing out. I shoot the water in the inlet and after I did that, I switch to the outlet. I did this about 30 times. 15 for the inlet and 15 for the outlet. This should fix your heater problem. That orange Dex cool is junk