So I was reading online that if you have errors with the active tank level sensor you may add some resistors to the wiring to trick your car into thinking the sensor is okay or the sensor may be covered in lots of dried up DEF and maybe giving it a wash would cause your check engine light to go away.
In my case my active level sensor went bad (code 4BAC) and upon reading online, BMW wanted to charge $2400USD to replace the active tank completely as they don't sell just the sensor. Did some more digging and found a post online where someone replaced the active tank sensor with a VW part on an x5d, but not on the 335d. Given the similarities between the 335d and the x5d I thought I'd give it a try too.
So I replaced the sensor with the VW part...and no more check engine light. It was around $330CAD+tax from the VW dealership. The VW part # is 7L6 198 970 A. There are two versions of this part make sure it matches the above and then its plug and play. No cutting or splicing wires of required.
Alternatively this sensor may be available here too:
http://xemodex.ca/ca/product/def-scr...it-for-bmw-x5/ for the part
Instructions
Step 1: jack up your car safely as you will start to work from underneath the car
Step 2: Lower the exhaust pipe from its rubber holders and unscrew exhaust protective plate about halfway on your car (4 large hex screws). Support the exhaust to prevent it from breaking
Step 3: remove the heat shield directly below the trunk I think size 8mm or 10mm nuts. In my case the nuts were really rotten and I replaced some with large galvanised washers and nuts afterwards
Step 4: you will now see the bottom of the active tank. There are two 17mm nuts you need to loosen that will let you get access to the sensor from the trunk.
Step 5: go to the trunk and remove the black metal plate under the rug giving you acess to the top of the active tank and sensor. There are around 10 size 8mm or 10mm nuts. You can't remove this plate if you skip step 4 since the 17mm nuts are holding the tank to the black metal plate in your trunk. Be careful of the antenna wire attached to the metal plate.
Step 6 unscrew 3 bolts using an allen key driver from the top part of the sensor
Step 7 unscrew the lid of the sensor by hand (or using flathead and hammer since mine had a ton of dry DEF)
Step 8 replace with the new sensor. It looks like a basket, will get a photo of it once I log onto a computer
Step 9 scan your car for codes and put your car back together
Lastly: some may be able to get this covered under emissions warranty depending on their state and mileage.
also to avoid from sensors going bad, I read that the solution is to not overfill the def tank.
I apologize if it doesn't make sense, I wanted to take pictures as I went along but my phone froze from the cold... So if you have any questions let me know