Mass flow - open circuit. It’s that pipe on drivers side that has the 2 sensors sticking up in between the two rubber hoses. If its bad, most likely the temp sensor inside was hit by moisture, ie, water. If you are not loosing coolant, then most likely was condensation which builds up in egr system after it has been ran all day and sits all night. If on the off chance you are loosing coolant and you don’t have any external leaks and can’t find where coolant is going, very possible your egr cooler is starting to leak internally. If you replace the mass flow assembly, pipe with the sensors and it’s small ecm, look and see if the “liquid “ inside is is clear or coolant colored and go from there. There are other reasons for it to code an open circuit, but that is the most common cause. Most of the time, you can run it with the code and it not effect engine op but it can cause odd power issues and even cause the turbo to go stupid intermittently. And before asking, it is an assembly and unless someone that I haven’t heard of yet has done it, you can’t change the sensors, they are welded to the pipe and that pain in the butt lil ecm and sensor are programmed together. Hope that helps