3dngin

I have a 2011 GL450 and when i turn on the car sometimes both red triangle indicators come on and stay on and never audibly warn me. Sometimes they don't turn on at all. I read other threads indicating the radar sensors may be dirty so I took them out to inspect them. Nothing seems out of the ordinary. It's very dirty up there, but the radar sensors, themselves, seem pretty clean and undamaged. Can someone help me figure out what is wrong?