Plugged Heater Core
WARNING:
- The heater core inlet hose will become too hot to handle if the system is working correctly.
- Check to see that the engine coolant is at the proper level.
- Start the engine and turn on the heater.
- When the engine coolant reaches operating temperature, feel the heater core outlet hose to see if it is hot.
If it is not hot:
- the heater core may have an air pocket
- the heater core may be plugged
- the thermostat is not working properly