There was a time that if your Ambient Air Temperature sensor wasn't working the only noticeable thing was your outside temperature display was incorrect or non functional. These days however the Ambient Air Temperature is factored in to the engine control logic and an integral part of the system. The system monitors the engine coolant temperature, the intake air temperature and the ambient air temperature. Cold air is denser than warm air and that becomes part of the air density calculation of the engine computer. It affects fuel mileage, emissions and performance. The pid for AAT isn't as straightforward as you might think however. You would expect that a vehicle sitting overnight without running would have engine coolant, intake air and ambient air temps all pretty close to each other when looking at the data with key on but that isn't always the case. There are rules of engagement that vary a bit from manufacturer to manufacturer that prevent the AAT pid from showing the current state of the sensor reading. Now the sensor can be working just fine and the sensor resistance has changed to match the current temperature but the number you are seeing with key on usually won't update to current reading until the vehicle is driving under the particular manufacturers rules. Some may update on the starter engagement but others not until the car is driving at some distance, some length of time or some speed. However the engine computer used the current resistance in real time to calculate the conditions at the engine start up. It just isn't showing the current value yet on the scan data. It is a bit like the computer logic that prevents your fuel gauge reacting to the fuel sloshing around in your tank while you are driving. You don't have to see the gauge reacting to that because the sensor data is being filtered and slowed before it reaches the gauge.
Bearing in mind that the AAT data you are looking at isn't necessarily what the engine control module is currently reacting to, know that the ecm is very capable of noticing a discrepancy that isn't rational if one temperature sensor reading is contrary to what would be logical compared to the others. If you want to see the current readings for your own comparison, you would meet the conditions required for the scan pid to update as the quickest and simplest way. The computer will set a code if it sees illogical temperature data but there are times when a sensor can be incorrect yet acceptable to the computer so it never hurts to check.
A few things of note, the intake air temperature is measuring under hood temps of air in the air intake tube so not the same as the outside air temperature being monitored by the ambient sensor which is often located near the front grill or in an outside rearview mirror. Also, the rules of engagement for the climate control ambient air pid are not the same as the rules for the engine control even though it is often using the same sensor, so it is not unusual to see different temp readings in HVAC ambient air and Engine ambient air. Temps are also factored in to the computer deciding how long your engine should take to warm up, a P0128 being related.
Thanks for reading!
Kenny@GGAuto.Repair
