Sometimes it takes up to 20 minutes of driving for it to start reading a temperature. Sometimes it works quickly. Is this a fault of some sort or just some funky behavior for it?

Doesnt really bother me as it really serves no function other than to look at it but just curious why sometimes it just hangs in various temps after startup.

 

This theory of operation from the manual may help to explain.

Circuit Description
The outside air temperature sensor is located in the RH front of the engine compartment, on the radiator support near the horn assembly. In this position, it is exposed to airflow 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 at the HVAC Control Assembly. The HVAC Control Assembly supplies a reference voltage (about 5 volts) on CKT 735 to the sensor and measures the voltage drop in the circuit to obtain a temperature signal. This signal is an input that tells the HVAC control head 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 242,700 ohms at -40°C (-40°F) to 2488 ohms at 60°C (140°F). When the outside (ambient) air temperature is cold, the thermistor resistance is high; therefore, the HVAC Control Assembly will receive a high signal voltage input. If the air temperature is hot, the thermistor resistance is low, the HVAC control head will receive a low signal voltage input.

The outside temperature is displayed on the display panel of the HVAC Control Assembly. The displayed temperature value is derived from a series of calculations performed by a processor in the HVAC Control Assembly. The calculations performed by the HVAC Control Assembly will use coolant temperature, vehicle speed, and ignition ON/OFF time to limit how fast the temperature display will be increased if a difference exists between the outside temperature sensor reading and the last value calculated by the HVAC Control Assembly for the temperature display. These calculations are used to eliminate any heat soak variations which may occur around the outside temperature sensor particularly after the engine has been turned off. If the car has been turned off for over 3 hours and then is restarted, the temperature display is immediately reset to ambient temperature conditions. If the car has been restarted within a 3 hour period after shutdown, the temperature display will not immediately reset. The temperature display will update after the car is driven between 32 and 72 km/h (20 and 45 mph) for 90 seconds or above 72 km/h (45 mph) for 60 seconds. In either condition, the temperature display will then be allowed to increase at the rate of 1.3 degrees Fahrenheit per second. Update rates are quicker at higher speeds because the outside temperature sensor reaches ambient temperature faster. If vehicle speed is not available from the Class II data line, it will be assumed to be at 40 km/h (25 mph). Finally, if coolant temperature is within 40°C (104°F) from the outside temperature sensor reading, the temperature display will update immediately upon vehicle start up. If the above conditions are not met, then the outside temperature display will not be allowed to increase.

A condition may exist were the outside temperature sensor is reading a lower temperature then the last calculated value stored in the HVAC control head. In this condition, the temperature display will always be allowed to decrease at a rate of 1.3 degrees Fahrenheit per second. Heat soak conditions in the engine compartment will not exist and thus do not cause a false lower temperature reading by the outside temperature sensor.

If the outside temperature sensor is shorted or not connected, the system will always read 13°C (56°F) as a default.