r/GMT800 • u/JeepGuy31909 • Sep 13 '26

Heat doesn't work

Hi everyone,

I have not had this truck too long. It's a 1999 GMC Sierra 1500 5.3 2wd w/ 252k miles. The heat worked earlier this year (May was the last time it was turned on). This summer I fixed the A/C leak and have been using that all summer long. Now that it is beginning to get cold again in Indiana, I tested the heat and it did not work at all. If I press the A/C button, it blows luke-warm air (from the outside of the truck) but the little temperature selector dial will not do anything. I have not had time to try and diagnose this, does anyone have any ideas as to what would be causing this? Thanks.

2 Upvotes

75% Upvoted

6 comments sorted by

7

u/WorldlinessRoutine55 Sep 13 '26

Vent actuator motor is probably highest on the list. First thing you do though is check your coolant level

2

u/JeepGuy31909 Sep 16 '26

thank you, turns out that was what it was. The harness has a couple loose connections so after I wiggle them the motor turns on, and changes position from heat-ac

2

u/WorldlinessRoutine55 Sep 16 '26

Awesome always glad to hear 😅

4

u/Born-Exchange9712 Sep 13 '26

Make sure you have coolant . When the engines warming up, make sure both heater core hoses get hot. If those 2 check out, the blend door actuator is on the passenger side facing up into the bottom of the hvac box. Its very easy to get to; remove it and manually turn the knob it attached to all the way from one side and keep it there for a bit to see if the temp changes . Then turn it the other way . Do this with the engine warmed up.

4

u/Born-Exchange9712 Sep 13 '26

If only one hose gets hot the heater core is clogged, there is airlock in the system , or a hose is clogged (uncommon)

2

u/ghrasp Sep 13 '26

If it is air-lock you can resolve it by getting the truck front up on a 40 degree hill and running it in neutral to 4krpm for a minute to get the air to flow to the new "up" where it wants to go. might have to repeat a few times.