I have a 2015 ML400 with 20,000 miles and today it reported "coolant level low". I checked under the hood and the coolant level is fine ... After the car is running and gets up to temperature, the error message goes away. After the car cools down (overnight), the error comes back.

Could this be a sensor problem?

 

Did you check the windshield washer fluid level?
I understand the error is pointing at the coolant level. Check the windshield washer fluid level... It's easy to check...

 

windshield washer fluid? Why would that show up as a "coolant" low problem?

 

Do NOT mess around with this warning. Get to the dealer service dept. You do NOT want a blown head gasket.

 

On w203 (c280), coolant level sensor and windshield fluid level sensor are connected in series. So, low of either of those fluids, light is triggered.

I don't know if that is the case with W166. That is why I said, check it. It doesn't cost you anything and you can check it in couple of minutes.

I typically try to check and eliminate the obvious and easy stuff first...

It's just a suggestion, you can decide what you want to do with that suggestion

 

Interesting ... the engine coolant and the windshield washing fluid sensors are in series? Wow ... that would make much more sense then as the windshield washer gets used much more and would likely be low. I'll have to check it.

Thanks

 

I've had this message before. Coolant level seemed OK but I added about a quarter cup of distilled water to the coolant reservoir and the warning went away. Seems the sensor is easily triggered.

 

I suppose that the water evaporates over a couple of years ... just enough to trigger. I thought it was a closed system - but probably not 100%. I'll have to check again more closely and add some distilled water. It's pretty full right now.

 

I heard back from the dealer (took the car in for routine oil change). The service tech told me the sensors for engine coolant and the windshield washer are completely separate. One does not affect the other. He said the coolant in the reservoir was a tiny bit low and they topped it off. No pressure leaks or any other issue.

The ML400 is two years old with 20,000 miles. I presume that in that time some evaporation of the coolant occurs even though it's a closed system. I don't think I'll see this warning again for another 20,000 miles.

 

Thanks for reporting back. Glad to hear you've resolved the low coolant issue.

 

Sorry, a little late to respond... Yes, coolant level is a bit low, even though looking through the reservoir looks good. Top up with coolant(1/4 cup) and all will be fine. Had this 2x now... negligible amount!! Sensitive sensor in reservoir.