My 2002 chevy malibu heater works fine. It just will blow cold air and then switch to hot air and the cold again. It never constantly blows hot air, it always alternates. Settings 1-5 on the fan work just fine. I just dont get constant hot air
Is your heater controls on "Automatic" or "Manual" Does it blow hotter with higher engine speeds and cooler with lower speeds? I'll need this information to narrow it down.
If it fluctuates with engine speed, this would indicate either a thermostat that is stuck in the open position (rare) or a partial blockage of the heater core.
If the fluctuation in temp has no correlation with engine speed, then this would indicate a failing control unit, sensor, stuck blend door inside the HVAC unit, or a leaking vacuum line/solenoid that controls the blend door.