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This post is a near copy of and totally inspired and influenced by Alex G's post:
Over-reading external temperature solved

Why fix/replace the outside temperature sensor? I mean you can just manually adjust the AC right?
Well my kids were getting travel sick on journeys and the car was simply stuffy even after a short journey.
Now that it's fixed the air in the car is fresh and the climate control is doing what it's supposed to do and
controlling the indoor atmosphere.
Also the fatigue from the constants drone of climate control fan on longer journeys is gone.

So this is what I did (again a near copy of Alex's post) and I wouldn't have attempted anything like this without his post.

The temperature sensor is basically an Negative Thermal Coefficient electronic component. Meaning its resistance decreases as its temperature rises. Typically the required NTC is 10KOhms at 25'C.
When I bought my car (second hand) the guy gave me a replacement outside temperature sensor. I got my mechanic to replace it. It actually made the situation worse.

Summary of what you need to do:
Take off the glass portion of the drivers side mirror.
Unscrew 3 Torx bolts.
Move the assembly around and pop out the sensor.
Dig out the expoxy resin including the old NTC from the sensor assembly. (I carefully drilled and drilled and used a sharp knife).
Solder in the new NTC.
Place and hold the new NTC in place fill the sensor assembly with new expoxy resin
Reassemble the inside of the mirror
Reattach the glass of the mirror

Taking off the glass:
Adjust the mirror all the way in and down.
I used a clothes peg to help give me a little more room behind the glass to get a screwdriver in. I can't stress enough to be very careful and gentle. I didn't crack the mirror but I was nervous as hell.
It's hard to get a good photo of how to get a screwdriver in to lift/move the white clips that hold the mirror glass in place.
940045

I used a narrow flat head screwdriver to lift up and push out the white clips below. It's quite tricky to get the clip up and back and stop it from snapping back into place. I managed to get the top one in this photo up and back, then the one closer to you in the photo (screwdriver going in horizontally) and then finally the lower one (screwdriver going in and down vertically. Once the 3rd clip with released the mirror glass popped out.
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Top clip:
940050

Second clip:
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Lower clip coming in from the top.
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How did I know the sensor in place was wrong and not a bad connection or something:
Take off the speaker cover on the inside of the drivers door (screwdriver in behind it and carefully pop it off).
I disconnected the cable going out to the mirror. Having removed the mirror glass I could trace the wires from the temperature sensor down to the following connector, I'm pretty sure its the two grey wires in this photo (polarity is not important):
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I measured the resistance of the (faulty) NTC and got 3.6KOhms. It should be 10KOhms (air temp was about 20'C).
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So how about the new NTC 11.6KOhms is more like it. Sorry picture is blurred but leads from the multimeter are going to either side of the NTC:
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I wired the new NTC going back towards the car:
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I made sure the mirror glass wires were not shorting to each other or any other metal and I turned on the ignition and using a my cheap (uncalibrated) Lidl outside temperature monitor (1.5'C error, I could live with that):
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The next few photos shows the sensor housing after I drilled and cut out the old NTC, its wires and metal bits. I didn't show the soldering of the new NTC into place (again polarity is irrelevant). The biggest issue was keeping the NTC and soldered wires in place and the assembly level so as to pour the new epoxy resin so that it filled nice and level.
(I'll try to add more photos in a reply to this post as I've reached the max number of attachments)

I tested that the new NTC is working correctly. (Note the temperature may read a little high as the epoxy is probably giving off heat as it cures)
I waited a few hours for the epoxy to set and started to reassemble the wiring looms and connectors and Torx screws.
Don't forget the mirror glass heater wiring. The mirror clips into place with a gentle push/press.

Now the climate control is very comfortable.....it's almost like a new car!!!!
 
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