1998 C280 with P0173- Fuel Trim Malfunction - Bank 2 CEL code

Yesterday my 98 C280 Check Engine light came on. Car seems to be running fine. I had AZ check the CEL and it showed P0173- Fuel Trim Malfunction -
Bank 2. Any suggestions? I checked the gas cap gasket and it appears ok. (although I have been told this would not cause that CEL code!) The light went back out today and is STILL out.
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