Touchy Temperature Control

Occasionally we receive a call regarding a Nissan that blows hot air only when the automatic temperature control is set to maximum heat. For example, if the temperature is set to 85 degrees Fahrenheit and then the temperature selection is dropped down one or two degrees, the temperature goes to full cold. This is usually caused by a defective temperature sensor. For purposes of this article, the vehicle described here will be a 2004 Nissan Pathfinder.

This vehicle uses temperature sensors such as an in-vehicle sensor and an intake sensor to input to an auto amplifier (control head) what the air temperature is inside the passenger compartment and inside the evaporator case. The auto amplifier uses these sensor inputs to determine the position of the air mix doors for proper control of the passenger compartment temperature. When the system is set to maximum heat, the auto amplifier ignores the temperature sensors, sets the air mix doors to maximum heat and the air blows out hot. When the temperature is dropped down from the maximum heat setting, the auto amplifier will look at the temperature sensors to determine proper air mix door position. If one of these sensors reads an open or short circuit (or a very inaccurate temperature), the auto amplifier will not be able to properly control the air mix door motors, resulting in incorrect passenger compartment temperatures.

If you encounter a vehicle with this problem, the first step is to perform a 5-step self-diagnosis of the auto amplifier.

Step 1 performs a check of the LEDs and segments of the auto amplifier. Step 2 performs a check of the input signals from each of the sensors and gives an indication if any of the sensor circuits are open or shorted. Step 3 performs a check of the mode and intake door motor position switches. Step 4 performs a check of the actuators. Step 5 will give the actual temperature detected by each of the sensors. Since we are looking for a temperature sensor problem, we are mainly concerned with checking self-diagnosis in steps 2 and 5.

To perform self-diagnosis, turn the ignition switch on. Then within 10 seconds of turning the ignition switch on, press and hold the temperature control OFF switch in for at least 5 seconds. You will know self-diagnosis has begun when all of the LEDs and segments illuminate indicating the system is in step 1 of self-diagnosis.

To move from step 1 to step 2 (except LE models), press the temperature control up arrow one time and release it. On LE models, turn the temperature control dial one click clockwise. This same procedure will need to be used to move through all 5 steps.

When the system enters step 2, a “2” will be displayed in the auto amplifier. If all sensor circuits are good, about 25 seconds after the “2” is displayed indicating the system is in step 2, a “20” will be displayed. However, if there is a malfunction in any of the sensor circuits, a trouble code will be displayed for the corresponding failed circuit.

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Code 21 or –21 indicates an open or a short in the ambient sensor circuit.

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Code 22 or –22 indicates an open or short in the in-vehicle sensor circuit.

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Code 24 or –24 indicates an open or short in the intake sensor circuit.

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Code 25 or –25 indicates an open or short in the sun load sensor circuit. Note: Code 25 is very common to set if self-diagnosis is performed out of sun light – for example, in a shop. A trouble light can be placed over the sun load sensor to see if that code will go away, but a sun load sensor circuit problem will not cause the failure referred to in this article.

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Code 26 or –26 indicates an open or short in the air mix door motor Potentio Ballast Resistor (PBR) circuit. The PBR is the position sensor for the air mix door.

This problem does not involve going into steps 3 and 4, but you will need to go past them to get to step 5. So to move from step 2 to step 3 (except LE models), press the temperature control up arrow again one time and release it. On LE models, turn the temperature control dial one click clockwise.

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When the system enters step 3, a “3” should be displayed in the auto amplifier. Again the same procedure needs to be used to move through all 5 steps, so use the same procedure to move from step 3 to step 4, and from step 4 to step 5.

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When the system enters step 4, a “41” should be displayed in the auto amplifier. When the system enters step 5, a “5” should be displayed in the auto amplifier.

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Once the system is in step 5, depress the DEF switch one time and release it. The auto amplifier should display the actual temperature of the ambient sensor. If in ECON, the display will show a negative sign in front of the number. When the DEF switch is depressed a second time, the auto amplifier should display the actual temperature of the in-vehicle sensor. When the DEF switch is depressed a third time, the auto amplifier should display the actual temperature of the intake sensor.

If either the in-vehicle sensor or the intake sensor temperature readings differ greatly from the actual temperature inside the passenger compartment and inside the evaporator case, this problem will occur. Before condemning the sensor, check the defective sensor circuit for a 5-volt reference to the sensor from the auto amplifier with the sensor unplugged and key on, and also check the sensor for a good ground.

The in-vehicle sensor is located in the dash, just to the right of the steering column. The in-vehicle sensor receives air from an aspirator that is located in front of the heater case. The aspirator produces a low pressure (vacuum) due to air discharged from the heater case. This low pressure causes passenger compartment air to be drawn into the aspirator through a tube over to the in-vehicle sensor. The intake sensor is located inside the evaporator case to detect air temperature after it passes through the A/C evaporator.

Please contact us if you have any questions.