Coolant level check

[tehskeo]

Hey e36 crew. My OBC tells me occasionally that I should check my coolant level when I turn the ignition off. I've checked it when cold and it's dead on the "Cold" line. Sometimes the warning comes on, sometimes it doesn't. I remember having the same warning come on about 6 months ago and I added about 100mL of fluid and it went away.

Is it just that I'm on the edge of when the sensor goes off? Is it possible to have a leak so slow that the system loses 100mL over 6 months?!?

Any help would be appreciated!