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Discussion Starter #1
Short story:
Steady overheat message on MFI even when engine is cold, cooling fans run on high during and after engine is turned off, then stop after 5-10 minutes.

Long story:
The thermostat does not close completely (probably not related to the problem though), never bothered me in city driving, engine always at normal 200F or so. I was on a freeway for a while, temperature around 160F which is expected with leaking thermostat. Suddenly overheat message came on. I took a look at temperature gauge - strait up at 200F. Probably jumped from 160 to 200 in an instant because I was pretty much closely watching it. Pulled over on a nearest service plaza, radiator hoses are lukewarm, engine is not very hot on touch when I reached metal parts. Fans running in after run mode on highest speed. Waited for some time with "rest" heater on just in case. Fans eventually stopped, I started driving with temperature gauge around 150F. Shortly after got stuck in traffic at 5km/h for a half an hour, temperature quickly reached 200 and stayed there. Later, at normal speed temperature reached 160 again (thanks faulty thermostat) and after about 10-20 minutes the same overheat signal on MFI and temperature immediately at 200F. This time I did not stop and reached my destination. The false overheat condition seems permanent now.

Initial investigation:
I found two two wire temperature sensors (06A-919-501): G62 on engine and G83 on a radiator but according to other posts there is a three wire dual thermo-switch 1J0-959-481-A located somewhere at radiator.

Any suggestions and/or comments?
 

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What engine do you have?

Someone on here indicated that the dealer would be able to tell which sensor was bad by computer diagnostic of some kind. If you are just going to guess and start replacing things always start with the cheapest / easiest to replace.

Outside of the overheat message, another chap had a sticking thermostat that was causing the fans to run...I would guess the ECU gets an implausible signal and everything goes awry.
 

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Get a VCDS scan, and log temps. Also, the dash gauge isn't accurate at all.

Also, where are you?
 

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Discussion Starter #4
Get a VCDS scan, and log temps. Also, the dash gauge isn't accurate at all.
I suppose the dash gauge is sufficiently accurate to diagnose a slightly leaking thermostat. I had several cars on which the condition (go on freeway and temperature goes down noticeable) was very pronounced and confirmed by repairs.

I do have a VCDS however an unknown no-name third party adapter has problems communicating with Touareg, while works perfectly with my older Audi. It establishes communication, and then quickly looses it. Sometimes I am able to get and reset codes but it did not work today.

I'll see if there is anyone around with a better, original, adapter, otherwise will go to the dealer to perform something like this.

But still, where the 1J0-959-481-A three pin thermo-switch is located (if present)? Its nowhere to be found on schematic.
 

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Discussion Starter #5
I managed to get my VCDS working up to the point of getting codes (nothing besides general cooling system code caused by a thermostat) and clearing it. False overheat message is now gone but its appearance continues to be a mystery, unless it is very nasty way to alert driver about lower then expected engine temperature.
 
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