You will have to find a urea specific sensor otherwise it would get corroded prematurely.

Something like this is a good stainless steel interface that can then accommodate a variety of sensors. Assume the tank is plastic? Not sure how thick, but I assume this could be epoxied into the tank, or a plastic block that was drilled and tapped could be epoxied to the tank and then this could be threaded in?

Seems like the main PITA would be the effort to access/remove the tank if needed.

Schneider at 225 4" inch Stainless Steel Temp Probe Sensor Bulb Well 1 4 1 2 NPT | eBay

Then you could slide one of these into the cavity. Fill the cavity partially with RVC and insert the sensor.

This assumes the sensor values are close, if not could be adjusted with a resistor. I assume the temp of the fluid is not ultra critical, probably determines how much is used, but a few degrees here or there is probably good enough?

Sounds like someone should consider a "kit" because it seems these are not uncommon problems.

Or this $43 one.Its negative temperature coefficient (NTC) thermistor with 20 kΩ NTC Thermistor.Not sure whats the spec of BMW.But i guess that values can be adjusted by using external resistors.

Stainless Steel Temperature Probe > Vernier Software & Technology

It may be worth looking through Rehingold see if anything comes up in terms of measurements.

Alternatively, again with Rheingold, you should be able to get a read of the sensor and do the temperature calibration that way if needed.

Sounds like you two know all this electrical lingo and should put a Urea Tank Fix kit together and we the electrically challenged ones will buy them to fix our diesels and stick it to the dealer :)
Assuming a regular DIY'er could do this job with some instructions.

If BMW uses a thermistor, you can probably figure out what size it is by measuring the sensor resistance with a meter. Typically, thermistors are rated at 25C (77F). I use 10k stainless sheathed NTC type 3 thermistors with and without wells.

I would be more than happy to supply one of these sensors if someone wants to give it a try.

I'm looking forward to seeing a fix it here.

That is great news to have someone who is familiar with these sensors.

Here's a few more info I was able to dig out.

Code 0046F9 is described as "Reducing-agent active-tank temperature sensor, signal"

The description is: "Sensor monitoring, active tank temperature sensor. The fault is set if the raw sensor signal (voltage) drops below the minimum limit value of 165 mV."

That would be great if someone with tank in hand can measure sensor resistance.Yes Bmw uses thermistor instead of thermocouple.

I tried to get info about active tank temp sensor with X5 hooked up with rheingold.It doesn't give resistance values instead gives voltage values.
eg. when the temp was 25 deg C,then sensor voltage was 1061mV.

My engine was still little warm that's why there was difference of 5 deg C.So in that way we can try using 10K,20K thermistors and can check which one gives temp value same as ambient temp.

Here is the screenshot.