DIY video on replacing a coolant level sensor on a bmw e36. This is a quick and inexpensive job, should take under 30 minutes. The main symptom that you have a bad e36 coolant level sensor is that your OBC will keep saying"check control" and "check coolant level". If you check your coolant level and its normal, this means that more than likely your coolant level sensor is no longer operating properly. 

E36 Coolant Level Sensor: advanceautoparts.rt8x.net/Lbz9V

What happens is that over time the bobber will break into pieces and float to the bottom, once this happens your sensor will report that you have a low coolant level.
Coolant level sensors can be purchased for $10 - $25 depending on if you buy it offline or from the dealership. 
BMW E36 Coolant Level Sensors can be replaced without having to remove the expansion tank, however if you pop the cap off your expansion tank and see a long hairline crack on the internal plastic, you should go ahead and purchase another expansion tank.
To replace the sensor, simply remove the bleeder screw, and the connector that attaches your expansion tank to the radiator. Afterwards remove as much coolant from the expansion tank as possible, I used a turkey baster. Remove the one plastic bolt holding in the windshield washer tank and get it out of the way. 
Raise the expansion tank as high as possible and remove the old sensor (its only hand tightened). Install the new sensor put everything back together, refill the coolant, and bleed the coolant system.

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