Google RTB error 501: Lost in the mediation waterfall
What Google Authorized Buyers says
Google sends 501 in ${AUCTION_LOSS} when the bid lost in the mediation waterfall. Google says it supports most OpenRTB loss reason codes, and 501 is the extra value.
Google Authorized Buyers reports this integer in the ${AUCTION_LOSS} macro on the bid's lurl. A search for Google RTB error 501 is this code.
What to change
Read ${WINNING_MEDIATION_ECPM} on the loss notice when Google fills it. The bid had to clear the auction and the mediation chain. The price that would have taken the impression is the higher of that mediation eCPM and ${AUCTION_MIN_TO_WIN}.