1. Inspect the heater core for damage, cracks, and water leakage.
If there is any malfunction, replace the heater core.
2. Visually inspect the fins for bending.
If there is any bending, use the end of a flathead screwdriver to straighten the fins.
3. Visually inspect the heater hose for deformation.
Repair with pliers if there is deformation. If there is any malfunction, replace the heater core.