INSUFFICIENT, ERRATIC OR NO HEAT
Turn ignition off. Check engine coolant level hot and cold. If coolant is at proper level, go to next step. If coolant level is incorrect, go to step 3.
Start and run engine to normal operating temperature. Carefully feel heater core inlet tube. If tube is too hot to handle, go to step 4. If tube is not hot, service cooling system. Turn engine off.
Ensure cooling system is filled to specified level. Perform cooling system pressure check. If system holds pressure, go to next step. If system does not hold pressure, pressure test heater core.
Start and run engine to normal operating temperature. Feel heater core outlet tube. If core tube is warm or hot, system is okay at this time. If core tube is not warm or hot, test for plugged or restricted heater core.
Check for proper coolant level. Start engine and turn heater on. When engine reaches normal operating temperature, feel heater outlet hose. It should be hot. If outlet hose is not hot, heater core may have an air pocket, heater core may be plugged, or thermostat may not be working properly.