1994 GMC Sonoma Heater does not work

When I had lose of heat in mine in was because the heater core was plugged. I've heard that the red coolant GM uses is more prone to gumming up. In anycase, I simply did an engine coolant flush. But, when u do it, be sure to disconnect the inlet and outlet hoses on the firewall that go to the heater core, and flush the heater core out thoroughly with a garden hose. When I did mine, I put a fine meshed strainer under the tubes to see what came out of the heater core. Little reddish rust coloured chunks. You should use your garden hose to flush out the radiator as well, and it's doesn't hurt to run some water through the tubes(while u have them disconnected any how), to flush out some the chambers in the engine block too. There are drain plug(s) on the engine also for draining coolant out of the engine block. Not a big deal if you don't remove the engine drain plug(s), but just be aware that there will be a some old coolant or water still in the engine coolant chambers if you don't remove the plug(s), and this will 'water down' your new coolant mixture some. Nothing worth worrying about, but I personally like to make my mixture more like 55% coolant to 45% water to offset the water remaining in the block. I live an climate that gets cold in the winter. Good luck.

Also, be gentle with the amount of pressure u use from the hose---use just enough pressure to get the job done.