My 2002 buick rendezvous is overheating. The water pump, thermostat, fan sensor and relay have been changed as well as the heater core. And the truck is still running hot. Could it be the radiator or have the head gaskets been blown.

Check the radiator isn't, blocked ,then check head gaskets for leaks by putting compressed air in the cylinders and see if bubbles come into the radiator with the cap off.

You have a needle in a haystack because so many things can cause your problem. I am leaning toward the main computer - which you can replace yourself for around $300. There are companies on the internet that will send you your pre-programmed computer, you have to supply your VIN number. Like many internet companies I always recommend that you are careful in your selection. Be careful to find a reliable company and call them on the phone to make sure they are a real company. If they do not have a phone number - find a different company.

Here are a couple things to try first:
1) Coolant sensor - but this usually makes your car run really poor because it can change the gas/air ratio. If your car is running great other than the overheating sensor - then this probably is not the problem. But worth checking.

2) Serpentine Belt - sometimes when you change the serpentine belt you can put it back on the pulleys out of order. Your car can run great but can cause overheating especially if you have the water pump turning backwards. Since you had the water pump changed you might just take a look at the belt to make sure it is wound correctly. There should be a sticker under the hood.

All said -- I am still leaning toward the main computer.

Check the radiator cap first, pressure test the cooling system next,, see where the leak is if you have one,, check the electronic fan switch and relay,, ,,, after that it would be the thermostat, check to see if the upper radiator hose gets hot before it overheats,, if not the thermostat may be stuck shut,,, good luck, and of course be careful if a hose blows or cap is bad you can get scalded by the hot water ,, it can be over 220 * f

I WOULD SAY NO BECAUSE WATER ONLY CIRCULATES WHEN YOU TURN ON THE HEATER, BUT IF YOUR VEHICHLE CONTINUES TO OVERHEAT IT COULD BE 5 THINGS. CLOGGED RADIATOR,RADIATER HOSES.THERMOSTAT,RADIATER FAN SENSOR OR THE WATER PUMP

Try connecting your fans directly to the battery to know if they are working. If they don't replace your fans. If they are ok, then change the thermostat.
Try this:
When engine cold, remove radiator cap, then fill the radiator with water and start the engine. If water jumps out of the radiator and continues going out as the engine warms up, then you have a bad head gasket and that's why is running hot.

head gasket it sounds like you have a warped head antifreeze burns white remove and have it milled and magnafluxed to check for cracks that will core your problem

i didn't see you last posting about your water pump but theres is a tiny radiator called a heater core. it has the same hot water running through it as your big radiator and engine have.so when you turn your thermostat to hot and kick on the fan the fan blows through the heater core and wallaa you have heat you probably need a thermostat.if you have no heat and it is overheating hear and there that's probably what you need.make sure you see if you filled the radiator up all the way when you changed the water pump.

Bad themostat, dirty radiator (the front of it), corroded interior core of the radiator, weakening water pump, bad radiator fan.....just to name a few. More than likely? Low antifreeze level, or the thermostat.

If the top hose is hot and the bottom is cold, your definitely not getting circulation. You,ve already done alot of work to this vehicle so you have pretty much answered your own question. About the only thing left is the heater core. Of course, I dont want to be the one to tell you it is definitely the heater core because obviously I am not looking at the vehicle. But If i was a betting man, thats where my money would be.