Sounds like it needs a recode - someone has done a swapsies

Magnet inside is misaligned?

Thanks for the continuing interest and replies. Bulk – can you please elaborate on what you mean by a “swapsie”?? As I mentioned, I replaced the tank and sensor with new, correct parts. How can just replacing these plug-and-play components affect or swap anything? Did I fail to do a procedure with the computer or something and the signal from the sensor lead wires got “swapped”? I’m a bit lost on what you are referring to. The reverse behavior I am seeing shouldn’t be caused by simply plugging in a new sensor, and there is only one way to plug it in.

Also, to do the “recode” – I assume you are referring to setting some flag somewhere – or “unswapping” whatever happened to begin with. Can you provide some detail? What is the parameter name and value and would I be using a GT1 or other software?

Terminator – you made me realize there is a test I could do; I could drain the expansion tank such that the float goes to the bottom and see if that makes the message go away. That would at least prove that there is indeed a magnet on the float and is working “properly” albeit backwards.

I just fixed this problem two weekends ago.

I did a complete coolant hose/waterpump job in December. And like many suggest, I also purchased a new Expansion tank.

From day one, I was getting this error. I checked the wiring over and over. I tried burping, and setting the water levels high and low, I changed the sensor twice.

Recently, I started leaking coolant out of the lower connector to the EX-Tank. I decided to get a new hose and another expansion tank. The leak went away and the low coolant level warnings did too. Bad tank.

Thanks for sharing your experience. So a "bad tank" really means that something is bad with the float system. I wonder if some of these aftermarket tanks are made without the magnet in the float? I mean a poor job of reverse engineering (instead of using a specification) would be to measure the tank dimensions and put a float in, but unless you really understood how the system worked, you would not embed a magnet in the float. Maybe that's it. The manufacturer would probably not get enough complaints to understand their tank was faulty since as we can see from the thread, figuring out why we get the message is not easily obvious. Just a theory...

permanent "check coolant level"message-how sensor works ?
 

Hi everyone,
It seems this is a long overdue explanation for how the coolant level sensor works on BMWs...
Everyone seems to agree the sensor is a reed relay that gets triggered by the magnet on the floater,the magnet on the floater causing the reed relay to close, right?!Only partially correct- But then, if the sensor is out of the dry-well or if the connector is disconnected, the "check coolant message" becomes permanent, also if the connector pins are shorted ( with a staple or something else), the message goes away!!! This seems to be "backward " !! So the explanation is simple:
The sensor should be "normal closed"- no check coolant message ! They achieve this, by having a SECOND magnet on the dry well !!!! This causes the reed relay( coolant level sensor) to close AS SOON inserted on the dry well! ( tested by me on an OEM tank). Now when the coolant level drops and the floater gets close to the reed relay, the magnetic field from the magnet on the floater will counter-effect the one on the dry well( sensor enclosure) to open up the relay= check coolant message !
This is how it works ! and this is why many after market expansion tank will throw this "check coolant level" message !- they don't have the second magnet on the dry well to keep the relay closed in normal operation !
OEM expansion tanks can fail also ( probably by this second magnet getting either loose or weak with the time- not being able to keep the relay closed in normal operation= coolant level OK, or by floater magnet falling on the bottom of the tank and counter effect the magnet keeping the sensor closed)
I hope this helps everyone understand once for all how this wicked system works!

If your sensor is reacting to the magnet ex situ, the tank is likely bad. Try another aftermarket tank, or fix it the first time and buy a genuine BMW tank from an online vendor.

Hi there, I know this tread is very old but how did you resolve the issue? Thanks..

Thanks for the excellent research and explanation. This seems to fill in all the mysteries in the rest of this thread. I'll try to remember it if I ever have any problems.

It just amuses me to see how many people don't know what or how a magnetic reed switch works. The Magnet operates the reed switch which is the coolant level sensor. It operates like a door alarm switch where you have a fixed magnet and a fixed switch. As the magnet gets into the proper proximity of the switch it either opens or closes the switch depending on if it's a NC or NO switch. Pretty easy circuit to understand. So it's not the magnet that is communication with the system but the magnet is operating the reed switch that then tells the system if the coolant is low or okay.