For 1990-2009 cars
HVAC Components
Temperature Sensors

### Circuit Description

The outside air temperature sensor is located behind the lower grille of the front fascia on the left side of the vehicle. In this position, it is exposed to airflow through the grill before the air enters the radiator. This provides a sample of the outside (ambient) air temperature.

The sensor is a thermistor used to control the signal voltage to the heater and A/C control. The heater and A/C control supplies a reference voltage (about 5 volts) on circuit PM 4 to the sensor and measures the voltage drop in the circuit to obtain a temperature signal. This signal is an input that tells the heater and A/C control how much cooling or heating will be required to deliver to the passenger compartment. It also affects the mode and blower motor speed.

Resistance of the sensor varies from 169,400 ohms at -40°C (-40°F) to 1,245 ohms at 60°C (140°F). When the outside (ambient) air temperature is cold, the thermistor resistance is high; therefore, the Heater and A/C control will receive a high signal voltage input. If the air temperature is hot, the thermistor resistance is low, the Heater and A/C control will receive a low signal voltage input.

The outside temperature is displayed on the display panel of the heater and A/C control. The displayed temperature value is derived from a series of calculations performed by a processor in the heater and A/C control. When the measured outside temperature is less than the displayed value, the processor filters less of the input signal, stores it in EEPROM, and rapidly updates the display. If the measured outside temperature is greater than the displayed value and the vehicle is moving, the processor filters more of the input signal, stores it in EEPROM, and updates the display at a much slower rate.

### Conditions for Setting the DTC

• | With the ignition in the ON position, circuit PM4 is checked continuously every 200 ms.

• | Circuit PM4 stays at or near zero voltage.

### Action Taken When the DTC Sets

• | The heater and A/C control displays the outside temperature as F

• | The system uses 10°C (50°F) as a default value.

### Conditions for Clearing the DTC

• | The conditions for the fault are no longer present.

• | Using a scan tool.

• | A history DTC will clear after 20 consecutive ignition cycles if the condition for the fault is no longer present.

### Diagnostic Aids

• | If the DTC is a history or an intermittent. Try to perform the tests shown while "wiggling" wiring and connectors, this can often cause the malfunction to appear.

• | Visually inspect sensor connector and harness for damage, corrosion or water intrusion.

• | Check for adequate terminal tension, mis-routed harness, rubbed through wire insulation, and broken wire inside the insulation.

• | If using the chart below, the resistance value of the sensor must be close to the ohmic values given in the chart. If not, the sensor is considered skewed and must be replaced.

°C | °F | Ohmsohms
--- | --- | ---
Temperature vs. Resistance Values (Approximate)
-40 | -40 | 169,400
-30 | -22 | 88,740
-20 | -4 | 48,580
-10 | 14 | 27,670
0 | 32 | 16,330
10 | 50 | 9,951
20 | 68 | 6,246
30 | 86 | 4,028
40 | 104 | 2,663
50 | 122 | 1,801
60 | 140 | 1,245

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. This step tests the reference voltage from the heater and A/C control.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the HVAC Systems Diagnostic System Check? | -- | Go to Step 2 | Go to Diagnostic System Check
2 |

1. Disconnect the ambient air temperature sensor connector.
2. Turn ON the ignition, with the engine OFF.
3. Measure the voltage at harness connector terminal A.

Does the voltage measure near the specified value?

5 V

Go to Step 3

Go to Step 4

3 | Inspect for poor connections at the harness connector of the ambient air temperature sensor. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 5 | Go to Step 6
4 | Test the outside temperature sensor 5 volt reference circuit for a short to ground. Refer to Testing for Short to Ground in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 5 | Go to Step 7
5 |

1. Install a scan tool in order to clear the DTCs.
2. Operate the system within the Conditions for Setting the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 8

System OK

6 | Replace the ambient air temperature sensor. Did you complete the replacement? | -- | Go to Step 5 | --
7 | Inspect for poor connections at the harness connector of the heating and A/C control. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 5 | Go to Step 8
8 | Replace and program the heater and A/C control. Did you complete the replacement? | -- | Go to Step 5 | --