How Do I Unblock a Car Heater Matrix?

dirty car image by Alexey Klementiev from

A car heater matrix, also referred to as a heater core, is a small device similar to a radiator inside the passenger cabin of a vehicle. A blocked heater matrix may cause the heater and defroster to stop working or to work inefficiently.

Causes of Blockage

Hot engine coolant flowing through the matrix is used to warm air for the heater and defroster. Engine coolant may contain debris from corrosion of metal components or deteriorating gaskets and hoses. This debris may clog the small openings in the heater matrix.

Removing Blockage

The heater matrix attaches to the engine through two heater hoses. One hose carries hot engine coolant to the matrix while the other returns the coolant to the engine. By removing each hose from the engine and flushing water into the outlet hose, water flows backward through the matrix. This flow may push the sediment out of the matrix removing the blockage.

Heater Control Valve

A heater control valve controls the flow of engine coolant through the matrix on some vehicles. A defective valve may also block the flow of coolant through the heater matrix. The location and operation of the valve will vary from model to model. However, valves are most often on the inlet heater hose and operate by vacuum pressure or cable.