Thermostat in a 2003 chevy venture van

if you stand facing the van follow the radiator hose on the right side the motor where it stop your thermostat is there

Usually a blend door problem, either the control or the blend door itself.

Change the thermostat and start from there.

If heat gauge reads normal, stays in the middle, then you do not need to mes up with the thermostat. I own a 2000 Chevy Venture and changing the thermostat was not fan.
It may be the Temperature Control Module, which you can buy for $45 and replace without problem, or may be an partially obstructed heating core. The GM Dex-Cool antifreeze, the read one, has additives, but if you do not flush the radiator every 2 or 3 years, can result in partially clogged heater core. I have done on my car twice till now, and replaced with green Prestone, on my own risk. Hope this will help. If need more information let me know. A have the Haynes manual for Chevy Venture 2000, and I am a car dealer.

Should be close to the thermostat housing.

all you gotta do is follow the top radiator hose to motor,it;s where the hose meets the motor.

distributor is advanced

Transmission SPEED SENSOR

under the intake plenum, which will have to be removed before you can access the thermostat.
I suggest having it done by a garage mechanic.

no you dont need a special tool you have to take the throttle body off it makes it sooooooo much more simple.