Explorer Coolant Issue

I had something similar happen with my old Opel Astra. Coolant reservoir was constantly low. Have you checked for any visible leaks around the coolant reservoir, radiator hoses, or even the radiator cap? Air pockets can also cause this.

Where exactly should I be looking for these air pockets?

They can be tricky to spot. Sometimes the system needs bleeding. Honestly, with the check engine light, you might want to get it checked professionally to avoid bigger problems down the line. Could be a sign of something more serious.

Thanks! Took it to the workshop, as you suggested. Turns out the coolant level was indeed too low and they found a small leak. Repair and new radiator fluid cost me 52€. All good now!