I recently had the Check Engine Light come on with the P205c/P205b fault codes. I did some extensive research because I did not want to spend $475 on a new part to fix this as my Passat just went out of warranty.
The failed temperature sensor is integrated inside the AD Blue Tank and you cannot change this sensor out at the component level. The part number that contains this sensor is 561-198-970.
This temp sensor monitors the temperature of the DEF fluid and if it drops below -11 Degrees Fahrenheit then it activates the heater inside this tank. Since I live in Texas I don't really have to worry about -11 degree temperatures here. There are two ways to fix this issue yourself if you do or do not live in a climate where the weather gets below -11.
before you start, purchase a 10k Ohm Resistor or Automotive grade 10k Ohm Submersible Thermistor, Solder, heatshrink.
Mouser Electronics or eBay are two places to find these components.
Once you have all your parts ready, Here are the steps to take:
Use the jack in your trunk to lift up the rear passenger side until the tire just barely comes off the ground.
Remove all the torx screws that are around the rear fender well.
There should be 5 of them if I'm not mistaken.
Pull out the plastic part of your lower bumper/quarter panel.
Just enough to give you a few extra inches for removing the tank.
The tank is held in place by 3 bolts, remove them and drop the tank down. (Be ready to catch it if it's full of DEF as it will be heavy)
Find the Wire harness that is secured to the filler neck of the tank.
Pins 3 & 4 are the temp sensor wires connected to the failed temp sensor inside the tank. They are purple/yellow and Brown and on the other side of the connector the wire colors are Black & White going into the tank.
Easy Fix option if you're lazy or live in a warm weather climate:
From the other side of the plug, cut the wires that are going into the tank (Black & White wires). Make sure to leave enough slack for the resistor or thermistor.
Solder & heatshrink a 10K ohm resistor or 10k OHM Thermistor to the cut wires still connected to the plug. Make sure that the connections are made back to the computer and not to the tank. Leave the wires from the tank cut off and just use some tape or heat shrink to protect them as they will never be used again.
For Cold Climate Areas:
Take a picture of the tank before you start, ensure you know the position of all the cables, hoses and top pump position.
Remove all the harness, Hoses and Pump from the Top of the tank.
Unscrew the Top of the tank and remove the rubber grommet and plastic circular housing from the tank. The circular housing is locked in, just twist it counter clock wise and it will become detached from the tank.
Find/trace the wires inside the tank that are associated with pins 3 & 4 on the outside of the tank.
Using a Submersible Automotive Grade 10k Ohm thermistor,
Solder/Heatshrink the thermistor to these wires and make sure the probe part of the thermistor is located in the small part of the tank with the other sensors. Some Drilling may be necessary.
As I live in Texas, I did not pursue or try the Cold Climate option I described above.
Also, as I was troubleshooting this issue, I actually drilled a small hole in the plastic and extended the wires so I could access them from my trunk when I was trying to figure out the correct resistor to use.
At the time I was not sure what resistor size to use. After trying different values. I borrowed a friends 2014 TDI and used my meter to find the value VW uses for this component.
The 10k Ohm Resistance reading is equal to 25 degrees Celsius or 77 degrees Fahrenheit. This resistance reading will make the computer think the DEF fluid is at this temperature.
In the pictures attached I show the connections to look for.
(See Links below)
If you don't want to drop the tank, another option is to cut the wires from the connector by the rear wheel and add the resistor or thermistor from there. It also uses pins 3 & 4 on one of the wire harnesses.
Once I added the 10K Ohm resistor, it took me about 4 days of everyday driving before my Check Engine Light went out. I'm not sure of the exact number but the computer needs to go through a certain number of "driving cycles" before it realizes the problem is fixed and the light goes off.
One more thing to note if your adblue tank is full, unplug the harness/hose and dump all the DEF fluid into a large clean container.
This will help out as its very hard to get the tank bolted back into position. Once the nearly empty tank is bolted back in place, just re-fill it as you normally would. I use the small Filler bottle from VW. I have a hole cut out in the bottom of it and use a funnel to re-fill that small bottle. Once its full I push the bottle down to let the DEF enter the tank. Repeat until your full....
I hope this post helps.
No sense in spending close to $500 when you can replace this with a resistor or thermistor that costs less than a dollar. (And an hour or two of your time)
Pictures to help you: