Manny, I got the OBD reader and there were acually 3 codes - 156, 161 and 173. I cleared them all yesterday and than ran the car for about 50 miles with the swapped MAF sensor, no CEL but when I read the codes again, it had 161 there again as pending code. So I cleared again and put the original MAF back, ran for 7 miles and no CEL came up. So I think that it is not the MAF, but rather that almost new O2 that was supposedly replaced a year ago by MB dealer. I will get underneath the truck tonight and measure if that Bank2, Sensor 2 is getting the right juice. Could the O2 be perhaps defective??