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You know this already, its for some option the vehicle didn't come from the factory with like fog lamps, heavy duty cooling efan thermostat, license plate light, optional horn, etc.
Did you follow the harness back to where it started out from?
 

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Thats not a cap, thats the ambient air temp sensor. you want to leave that plugged in... :)
Always wondered where that ambient air temp sensor was located at. (y)


Btw. Is this ambient air temperature engine related or is used to show the outside temperature in the dashboard?
Cause I've noticed recently that the outside temperature in the dashboard gets "stuck" quite often. Taking up to 15 min to update. Is there a fix for that?
Thanks
How do you know its stuck?
I noticed the amb. temp reading takes a while from leaving my overnight cool garage into the hot afternoon outside before it reads the correct outside temperature. Never paid attention to this so never timed it but i'd guess maybe 5 minutes maybe more or less.
I'd think the PCM has better more higher priority things to do than updating the ambient air temp.

It also depends on the exact component used to measure the temp and other factors.
There are thermistors, RTDs, thermocouples, semi-condutor, etc. components used to measure temperatures.
All have differing response times, some faster, some slower again depending on the parameters they're subjected to.

I've used thermocouples extensively to measure the heat rise of IC chips and other electronic components during my working career.
For example, I normally epoxied a thermocouple to an IC chip case to get a rough idea of the chips heat dissipation when its exposed to the max. operating ambient temp. spec.
It generally takes 5 time constants for a thermocouple to reach the actual temperature desired to measure depending on the actual ambient/internal thermal differential, its rate of change and other factors.

I'd imagine for vehicles they use an RTD to measure the outside amb. temp.
It also takes 5 time constants to reach the actual temperature and how the its encased.
Plus RTD's cannot generally be grounded maybe the reason its encapsulated in what looks like an electrically insulated plastic case?
For this reason alone it could take a while for the actual ambient temperature to reach the RTD element through all that insulation.

All this 'could' be a reason the PCM takes periodic time samples of the thermal sensor.
 
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