Hello Scotty, A friend has an 04 Altima and no heat. It has a 2.5 lt. engine and 115,000 miles on it. All of the interior controls seem to work as air blows freely at feet to defrost but is barely warm - at idle or speed. This model has 2 thermostats - a lower and an upper. The temperature gauge reaches operating temp and levels off at proper level. So I think that the lower thermostat is operating fine. There are no engine codes stored on the computer. I was considering changing the upper (thermostat) water control valve and temperature sensor. I checked and topped off the antifreeze with 50/50. Is this the best next step or am I on the wrong track. Thanks for any input. Hope you're enjoying your new location.
Try finding the temperature blend door and see if it’s stuck. If you find remove the actuator and move the door manually to the hot side see if that’s improves the heat. If it does then your blend door actuator is could be bad or lost it’s calibration with the hvac module.
Also found this TSB: copy and google it. Maybe it will help you.
TSB #NTB02-047c
Check out the heater topic in our FAQ
If there be no codes, gauges and cluster work, and no leaks in any of the coolant lines, I would move away from the thermostat and start looking into the heater core.
Thanks BruceRee, Do you know how you can determine if the upper thermostat is working? Can it stay closed and the car still operate properly? In the meantime I can flush out the heater core and see if water circulates or not. Thanks again.
The upper one has a different and suppose to be hotter opening temperature compared to the lower one. On a technicality yes, you can run it with the upper being stuck but will have a chance of overheating if you are reving a lot or driving the car pretty hard, or driving in summer. Cause that's when the upper is suppose to open to allow more flow. From what I understand at the least. The way I test it is by feeling if there be any cold spots on the hoses after 10min of idle to operating temp and 5 min of rev to 3k. If there still be cold spots somewhere, then the upper thermostat be stuck. And by feel around I don't mean with your hand, with a heat sensor. Don't wanna burn your hand unless you know exactly what you doing ya know haha.