2002 sierra v6 heater won''t get warm. fuses all good. water not leaking from core. I circulated water thru core via hoses under hood. now what?

Feel the heater hoses under the hood where they go thru the firewall. Both should be nice and hot if the heater core is allowing liquid to flow through it. If only one hose is hot and the other is only warm you need to remove both hoses and blow backwards into the core with compressed air to free any blockage inside of the core.

Most likely it is the heater control valve that shifts air from blowing across your heater core to your air conditioner core.

If your fan works and your car is fairly new..meaning your heater core isn't dirty and full of craap...then it is the climate cotrol valve.
If you are lucky, it might be a thermostat that is bad and blocking hot water from entering your heater core. Bottom line, if hot water doesn't circulate thru that little heater core, you won't get heat.
You might be able to look under the hood and grab the hot water lines that go thru the firewall and feed your heater core. If they feel cold to the touch after engine has warmed up, then you aren't getting circulating water to the heater core. If they feel nice and warm, then the heater core is fine, but your climate control valve has closed off the fan that blows across the core to disperse heat into the cabin

It sounds like the bypass valve is stuck open. Check bypass valve and vacuum hoses attached to it.
Hope this helps.
Bob

check the controls for the vent doors.

plugged heater core would be my suspect.

There should be a valve on the heater hoses leading to the heater core in the engine compartment or under the dash that would allow hot water to the heater core check this valve before changing out any more parts.

Your heater core has a leak in it and needs replaced.
Do it now before all your coolant ends up inside the truck.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/sean_54c3a3dc48a7773c

if u can try and take a the heater core hoses off under the hood and take a water hose and run water thru the heater core on both ends the in and out cause there could be junk in there causing it not to flow properly

i don't know realy if there is a fuse (though i doubt it) but issues with heatting may be this will help.

If the carpet under the heater core is damp, or if antifreeze vapor or steam is coming through the vents, the heater core is leaking.
Remove it and install a new unit (most radiator shops will not repair a leaking heater core.
If the air coming out of the heater vents isn't hot, the problem could stem from any of the following causes:

• The thermostat is stuck open, previnting the engine coolant from warming enough to carry heat to the heater core. Replace the thermostat.
• There is a blockage in the system, Preventing the flow of coolant through heater core. Feel both heater hoses the firewall. They should be hot.if one of them is cold, there is an obstruction in one of the hoses or in the heater core,or the heater control valve is shut. Detach the hoses and back flush the heater core with a water hose. If the heater core is clear but circulation is impeded, remove the two hoses and flush them out with a water hose.
• If flushing fails to remove the blockage from the heater core, the coremust replaced.
  

the heater core is leaking.