If your temperature gauge is lower than normal, suspect a thermostat stuck open. If the gauge reads normal, feel the two heater hoses. After driving on the highway for 15 - 20 minutes, the hoses should be too hot to hold onto for very long. If they are cool, suspect a plugged heater core. They can usually be flushed out with a garden hose, but don't put more than 15 pounds of pressure on it. If the hoses are hot, suspect a problem with the actuator for the temperature door or its linkage, or the computer module that runs them. (We used to do this with cables and levers, not over-complicated, unreliable, unnecessary computers)!