SRS Light has me puzzled

[RES]

1995 C220. SRS light stays on for a minute after starting car. I purchased a TRISCO PS-411 scanner to trouble shoot the problem. Read codes 24, 139. Replaced drivers side seat belt switch. Erased code but light stays on and code 139 comes back. I am puzzled by code 139. It is not on the list for the SRS function. Problem seemed to occur after I put a large heavy box on the folded rear seat. After doing a search on this web site, I checked resistances on the old and new seat belt switches. They both read the required normal values latched and unlatched, so it seems that the original switch was OK.