First look at the temperature gauge on the dash, if you have one. Is that in the normal range? If it's lower than normal, the likely suspect is the thermostat is stuck open. You can verify that first by feeling the upper radiator hose after driving for a while. It should be too hot to hold onto for very long. Your mechanic can also connect a scanner to view live sensor data. The coolant temperature sensor will be displayed in voltage and degrees. It should be around 190 to 210 degrees Fahrenheit. If it's closer to 160 degrees, the air from the heater will not be hot enough.
Next, if everything points to the coolant temperature is acceptable, feel the heater hoses. Those also should be too hot to hold onto for very long. If they are not, but the radiator hose is, suspect a plugged heater core. That can usually be cleared by disconnecting one of the hoses, then running water through it from a garden hose.
If the heater hoses are sufficiently hot, but the air from the heater is not, that points to an actuator or temperature door problem with the heater box. That can get real involved and is best left to a mechanic who specializes in air conditioning and heating systems.
Tuesday, November 17th, 2015 AT 3:34 PM