Heater rarely works when it does work it does not blow very hot

Welcome to 2CarPros.

The first thing to check are the heater core hoses. With the engine running and at operating temp and the heater on high, see if both heater core hoses are hot. If they are, then we have flow through the heater core. Note that one will be slightly cooler (the return line).

If you find that both hoses are hot, then you most likely have a problem with what is called an air mix or blend air door. The door is what determines the air temp entering the vehicle.

Now the good news. On this vehicle, the air temp door is mechanically operated by a cable that connects the door to the temperature switch on the dash. The door is located under the dash to the right of where your right leg would be. I attached a picture of the actuator and the directions for adjustment. Before going to the trouble of adjusting it, make sure when you adjust temp at the controller it moves.

Here are the adjustment directions. Often times, the cable mount will come loose and allow the cable to move rendering the it not functional.

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2003 Nissan-Datsun Sentra L4-1.8L (QG18DE) ULEV
Air Mix Door Motor
Vehicle Heating and Air Conditioning Air Door Actuator / Motor Adjustments Air Mix Door Motor
AIR MIX DOOR MOTOR
Temperature Control Linkage Adjustment

pic 1

Temperature Control Cable
- Turn temperature control knob to the full cold position.
- Set the air mix door lever in the full cold position by hand.
- Pull on cable cover in the direction of the arrow and then clamp cable cover.

After positioning air mix door control cable, check for proper operation.

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Let me know what you find.

Take care,
Joe