The controls are operating incorrectly.

Advise the operator about the proper operation of the controls.

The engine coolant level is low.

1. Add the coolant as needed. Refer to Cooling System Draining and Filling in Engine Cooling.
2. Verify that there are no coolant leaks.
3. Run the engine in order to clear any air locks.

The engine thermostat is faulty.

1. Verify that the thermostat is operating properly.
2. Replace the thermostat as necessary. Refer to Engine Coolant Thermostat Replacement in Engine Cooling.

The heater hose has a kink.

1. Verify that the hose is kinked.
2. Straighten the kinked hose.
3. Replace the hose as needed. Refer to Heater Hoses Replacement .

The heater core tubes are obstructed.

An obstruction usually causes a squishing noise in the core.

1. Remove any foreign material from the core.
2. Replace the core as needed. Refer to Heater Core Replacement .

The blower motor or the blower circuit is faulty.

1. Repair or replace the faulty wires and connections.
2. Replace the blower motor. Refer to Blower Motor and Fan Replacement .