1. Low coolant level: Make sure the coolant level in the reservoir is between the "Full" and "Add" marks. If it's low, add a 50/50 mix of antifreeze and water.
2. Faulty thermostat: The thermostat may be stuck closed, preventing hot coolant from flowing to the heater core. To test the thermostat, start the van and let it run for a few minutes. If the upper radiator hose is hot but the lower hose is cold, the thermostat may be stuck closed.
3. Clogged heater core: The heater core may be clogged with debris, preventing hot coolant from flowing through it. To clean the heater core, flush it with a mixture of water and vinegar or a commercial radiator cleaner.
4. Faulty heater control valve: The heater control valve may be stuck closed, preventing hot coolant from flowing to the heater core. To test the heater control valve, turn the temperature setting to "Hot" and feel the heater hoses. If the upper heater hose is hot but the lower hose is cold, the heater control valve may be stuck closed.
5. Faulty blower motor: The blower motor may be faulty, preventing hot air from being blown into the cabin. To test the blower motor, turn the fan speed to "High" and listen for the sound of the motor. If you don't hear the motor running, it may be faulty.
If you're unable to diagnose and fix the problem yourself, take your van to a qualified mechanic.
