I have a 2004 GMC sierra 4x4.heater and air conditioner thermostat doesn't respond consistantly,also engine temp gauge just started to run hot.read out says engine coolant is hot even soon after i start the truck. can u help?

you might want to check the wires going to the temperature sensor on the engine make sure it hasnt rubbed the protective coating off, this can give you a false reading, the safest way is to put a manual gauge in place of the temperature sensor and run engine to make sure the engine is not overheating. You can pick up a cheap manual temp gauge at your local parts store and install the temperature probe into the hole where the temp sensor was located and run the engine. It sounds like you have a guage malfunction. But is better to be safe than sorry.

Before assuming the heater core is blown check the following.

1. Check coolant level while engine is cold.
2. Leave the radiator cap off and let the engine run. See if you can see the coolant circulate
when the engine is brought up to temperature.
3. Check the temperature gauge inside and note if the engine temp is correct.
4 Does the engine overheat?
5. Feel the heater hoses and check if they get hot.
6. Can you smell antifreeze inside the cab?

Start here and let me know.

Air pockets will make the guage do that, and also cause your heat to come and go. Also, a poor radiator, as in rotting, or plugged up, will cause high temperature issues.

My truck just did the exact same thing -- and it was a Sensor -- cost me $40.00 to get repaired at a local auto repair shop.

THE 1ST THING U DO IS GO UNDER THE HOOD WHEN ENGINE IS HOT, CHECK THAT BOTH HEATER HOSES (INLET AND OUTLET) ARE GETTING HOT, IF ONLY ONE IS HOT FLUSH THE CORE, IF BOTH ARE JUST WARM THEN THE THERMOSTAT IS DEFECTIVE IN THE ENGINE, REPLACE IT, LET ME KLNOW WHAT YOU FIND.

sound like blend doors are not working there is a motor under dash or vacuum line under dash ant working they should be color coded red yellow black i think need more let me know

there are 2 coolant sensors one for gauge and one for ecm

check your coolant level on the radiator

It may be the thermostat

No your thermostat is stuck open which will not hurt the engine. but needs to be replaced, the thermostat is what allows the engine get up to operating temp. before allowing water to flow through the radiator and when it is stuck open it doesnt allow the engine to reach operating temp. so the heater never gets hot. I hope that helps you out.