How to use a multimeter to test if the car heating system is working properly?

Turn off the car and let the engine cool down before beginning the testing process.

Locate the heater control valve, which is usually found near the firewall on the passenger side of the engine compartment.

Set the multimeter to the ohms setting.

Disconnect the electrical connector from the heater control valve.

Connect the red probe of the multimeter to one of the pins on the electrical connector and the black probe to the other pin.

The multimeter should read between 20 and 200 ohms. If the reading is outside of this range, then the heater control valve may be faulty and should be replaced.

Reconnect the electrical connector to the heater control valve.

Start the engine and turn the heater on to the maximum setting.

Wait for the engine to warm up and check the temperature of the air coming out of the vents. It should be hot.

If the air coming out of the vents is not hot, then there may be an issue with the heater core or another component of the heating system that requires further diagnosis.