I have a 1999 gmc jimmy 4.3 L V6. I recently bought this truck and have had some minor problems with it but that is expected for the year. Lately the weather is starting to get colder. 0 Celsius over night, and now my heat stopped working. By this I mean the heat inside the cab of the truck. I recently changed the thermostat to see if this was the issue but it did not help. The coolant level is full. After running for five minutes the top radiator hose gets hot. The fan inside the cab works but it just blows cool air. I am a student so I'm broke most days. Any suggestions would be appreciated.
It is likely a heater core or as mac suggested a diverter valve of some type.
Do you smell anything unusual when you run the heat now? If yes, it points more to it being a heater core issue. I believe you have to completely remove the dash to replace the heater core on your vehicle. So, it would not be cheap if you take it to a shop but also time consuming if you do it yourself.
Therefore check it there is a valve that might be the source of the problem before you tackle the hard item.
Used a garden hose with high pressure nozzle into the heater core outlets on the firewall and ten minutes later great hot air again
This might help if you have a clog. Make sure to blast the water backwards else you may embed the clog deeper into the heater core.