Chevrolet k1500 heater

SOURCE: 1996 ford contour gets hot , no hot heater or temp guage movement

Is your temperature gauge working now? That is of more concern then your heater as you wouldnt know if your engine was overheating until its too late. Your heater core could be clogged, they get clogged quite easily. You can flush them or replace. The part is inexpensive but on some models you have to take out dash to replace them, but on other models it only takes about 30 mins and a few bolts.

SOURCE: heater blow cold air in my 1994 Honda Accord (4

Yes clogged heater core may be the answer in your case .

SOURCE: Heater doesnt work blows cold air, replaced

This is what I did to solve the exact same problem.I got a pump that you can hook to a drill that has garden hose connections on it.I made 2 hoses to pump into my heater core and hook to the pump. I bought some radiator cleaner at Auto Zone and put it in a 5 gallon pail with about 2 gallons of hot water.I pumped the cleaner from the pail into the OUTLET side of the heater core.(By going in the outlet side you are back flushing the core.) I put an old cloth over the top of the bucket and let the outcoming cleaner run onto this cloth acting as a filter for the dirt coming out of the heater core and let the pump run about 10 minutes. Believe me,A LOT!!!!!!!!!!! of dirt,rust etc. came out of my heater core. The heat now works excellent! Good luck.