With the engine at full operating temp, locate the 2 heater hoses where they go into the firewall and feel them to see if they are both hot to the touch. If not too hot to hold, then you have to look at the actual engine temp and if that is up, if it is, then your looking at a restricted heater core.
If you find that they are both too hot to hold, then the heat in that vehicle is controlled by a blend door that regulates heated air flow. it is operated by an electric motor/actuator. The problem can be that the actuator is stripped or inoperative or the door itself could be damaged. This is what needs to be determined by examining the actuator and see if it is responding to heat change commands or not.
If they are both not too hot to hold on to, then you either have a restricted heater core or the system is not completely full and getting air pockets for some reason.

Answered by Wrenchtech (expert)
14,708 answers provided

Thanks, very helpful. Hoses near firewall feel about same temp as upper radiator hose. Owners pamphlet says one fuse block (of 2) is on passenger side under instrument panel... cannot find looking under glove compartment. Ditto on RH side above accel pedal. Is the "instrument panel" something other than the dashboard?

Response from dummkopf352
5 questions asked

No, instrument panel and dash are the same thing. It sounds like you're having a problem with the blend door operation.

Answered by Wrenchtech (expert)
14,708 answers provided

the 1st fuse box is on the very end of the passenger side instrument panel, you have to fully open the door to get at it. The 2nd is under the hood on the passneger side in the black bos that has the jumper peg sticking out.

Answered by surfcity
0 questions asked

Thanks... The 1st sounds like the way Generous Motors located a fusebox on my Chev 2002 Silverado 2500HD (except maybe for right vs left). Appreciate your help!

Response from dummkopf352
5 questions asked

Closing out question

Answered by Wrenchtech (expert)
14,708 answers provided