We just bought a 1999 GMC Jimmy and the heater doesn't work. The fan blows but there is no heat. The air conditioner works fine.

Heater core stoped up.pull both hoses flush with water hose, one side of pipe then the other.be sure to have good pressure on water hose

If you ac is working all the time even when dial switches to hot side try the blend door motor, i had this problem switched motor works fine now

Could be that the heater core is disconnected, there are two hoses that attach to the heather core that come out of the engine, this provides hot water to the core thus making the heater work, when the heater core starts leaking water the quick and fast fix would be to bypass the heater core and connect the two hoses together, check under the hood on the left passenger side there should be two hoses about 1" thick see if there are connected together or of they seem to be connected to 2 outlets that come out of the inside of the vehicle. people do that specially if selling vehicle.