How to Know if a Heater Core Is Damaged

1. Raise the hood of the vehicle. Remove the radiator cap and check the coolant level. If the coolant is low, on some vehicles there is not enough to circulate through the heater core. On others this won't make much difference. However, make sure that there is enough coolant and that the engine is cold for this test.
2. Crank the vehicle, set the heat to full, and let the engine warm. Inspect the heater hoses going from the engine to the firewall. Feel the hoses. There may be a heater control valve in one of the hoses. Feel before and after the heater valve, which should be hot on both sides. If it isn't, the valve is probably defective. Check the manufacturer's procedure for testing the valve. If these hoses never get warm, the engine is running cold and the thermostat needs replacing.
3. Feel both of the heater hoses that go into the firewall. The engine must be running at the proper temperature and the heater control set on full hot for this test. Both hoses should be hot. These hoses allow hot liquid to circulate through the heater core. If there is no circulation, the heater core is blocked with mineral deposits and must be replaced.
4. Look closely at the hoses and clamps that connect to the heater core. Inspect the hoses at the engine and any hoses, pipes or junctions leading to the overflow container. These should all be dry. Repair any leaks that would prevent adequate coolant flow to the heater core.
5. Shine a flashlight under the dash and look for coolant leaks. Coolant has a distinct odor. If there is a coolant odor but no leaks, this could indicate seepage that may be stopped with a cooling system sealer. If there are drips, the heater core must be replaced.