Help me w/OBD II codes 161,173

[mslc791]

I am really confused, about a year ago, the dealer replaced left rear O2 sensor, the truck had codes 156 and 161. About a month ago, CEL came on and is usuallu on for 2 days then goes off for maybe 1/2 a day and then comes back on again. I checked the almost freshly replaced O2 sensor's wiring, it seems OK. So I thought the MAF might be defective. I swapped the same unit from my other MB and the on/off CEL game continues as before. So if it's not MAF, where should I check next???