Obstructed Coolant Flow

Possible causes of insufficient engine coolant flow are as follows:

• Improper heater hose routing.
• Low engine coolant.
• Pinched, kinked or plugged heater hoses.
• Plugged heater core.
• Plugged heater hose supply and return ports at the engine.

If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.