1998 Jimmy heater problem

Hello all, This morning I tried to use my heat. I get warm air from the vents for about 15 seconds and then it goes cold. I moved to control switch back to off and then to vent, same thing.
Any thing quick to look for? Could it be the heater core?
TIA, Dave-
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Mine did that when I was low on fluid. I went 4 years with a leaky radiator.
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I had the same problem with my 98 Blazer. The heater core was plugged. I flushed out the radiator & core & replaced the GM Crap with Prestone Yellow back in October. I get plenty of good heat now

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