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Is the DEF tank heater always on? I noticed a few weeks ago that my outside temperature sensor was off by about +20 degrees. I'll be driving in below freezing weather this winter and wanted to see if the outside temperature sensor could cause the DEF to not be heated when it needs to be. Asked the dealer about getting the sensor replaced and they said it would be about $200...needed to drop the front bumper to get to it.
 

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Takes about 15 minutes to change the sensor.....you can see it in the side of the fog lamp housing or block off depending on how the car is equipped.......looks like a recessed button from the outside.
To exchange, you just drop the horizontal panel on the bottom of the bumper, reach up and squeeze/pull the sensor out and disconnect the wire harness.....install is the reverse.
Keep in mind, once the car is started it takes about five miles for the temp to be updated and if it was where the sun was shining on it or over warm ground, it will read high till exposed to a ambient breeze for that many miles.


Before replacing anything though, disconnect the wire at the sensor and leave it off for a day......you will get weird readings from the display and the A/C won't work during this time. Reconnect and drive the car......you may find that it is once again accurate after five or so miles......the automotive equivalent of a 'reboot' of sorts.

Rob
 

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So the outside air temp sensor reading triggers the DEF heater?
 

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So the outside air temp sensor reading triggers the DEF heater?
I am not positive of that, but the OP is seeing a possible correlation and it seems plausible to me. There is no other source for ambient temperature and no need to power the def heater during warm weather, sooooo.
If nothing else, the OP will eventually get reasonably accurate readings on the display.

Rob
 

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I know there is a def temp sensor, as it reads out on my cts. I would guess it would use that sensor to turn on and off the def heater. I haven't looked it up, but that would be my guess.
 

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DEF temperature sensor is a part of the DEF Reservoir Assembly. DEF heating is controlled by that internal sensor and not by the Ambient Temperature Sensor. Since fluids warm and cool much more slowly than air, and because the tank is buried low and deep in the car where it is moderately insulated from the outside world, there would be a lot of difficulty accurately controlling the heater by ambient temperature.
 
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