Getting "Check Coolant Level" when 2019 MB GLE 400 car first started in the morning after parked outside overnight.

When this first happened, the coolant in the smaller, secondary coolant tank was indeed very low. No sign of a leak. Nevertheless, after driving it for a while light went off even though still too low. Topped it up with Zerex G40 pink coolant.

Next day, coolant warning light again on in the morning, though coolant reservoir only slightly low. Topped it up the rest of the way to the upper MAX indicator. Light went away immediately.

Next day, coolant warning light again on in the morning, this time no sign of low coolant in the tanks. Ok, is the car tooling with me? Added a small amount of coolant to the regular engine reservoir that was already filled to the line (is that a MIN or MAX marker line?) just in case, but warning light doesn't go out immediately. It did go out after a short drive, however.

I'm guessing from all of this that we actually have an issue with the coolant level sensor or the wiring to it. Of course this happens just after the CPO warranty expires! At any rate, where is that level sensor on the coolant tanks? Why does this only happen first thing in the morning? Anyone else seen the level sensr fail in this intermittent/temperature-related kind of way? Difficult to get a shop to diagnose an error like this because once the light goes out, nothing appears wrong.

This happened to me once a few years ago. It was adjusting the coolant level to the absolute perfect level, and my light has gone out and never came back. I hope this helps you.

So you checked the smaller tank under the engine cover. Did you check the main reservoir tank to see both tanks are filled?

Troubleshoot the sensor. I would see if you can clean the pins, harness plugs and check to see if both tanks coolant level off.

You fill up to the seam pretty much and thats the correct amount. the incorrect amount is like BARELY less than the "ok" amount it's a mercedes thing. You can go a little more for a safety cushion. I doubt anything is wrong with your car, thermostat goes out at like year 7 on these just look for a thermostat gasket leak its the first issue but I promise you mechanically youre probably ok. Sensors don't really go out unless the cars are really old/beat on. I've never needed to replace a sensor until the reservoir needed replacement.

Seems like you're correct, the coolant level in my main reservoir was actually low! It certainly was not filled up to the black! I thought the white bar on the outside of the coolant reservoir tank was the max level line, but it seems like that is actually a MIN level line.

Checked this morning that the car was giving the coolant low indicator this morning before even turning on the engine. Added coolant. The indicator went off just like that! I guess my coolant level warning system is working correctly, coolant was low but as the coolant warms up, its the level rises and it is no longer reading as low.

No idea why coolant in both coolant reservoir tanks got low. Seems to be a very slow loss of coolant over time. Also, it's important to recognize that there is no low coolant level warning at all for the secondary coolant expansion tank in the center by the engine (intercooler coolant system?), you need to visually check it from time to time (mine was extremely low and I had thought that was the reason for the indicator)