That code is for you passenger rear O2 sensor, check if it's snapped together completely and if wires are connected to the connector, may have a short from that mil, or something.. It may be nothing, but I do not think mil always clear codes but I'm not sure. Just check the mid switch then around and see if the code follows to the other bank.
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2004 Mustang GT 5 Speed
Full Bolt Ons
12.1 @ 117.9 (1.7) in 1/4th
7.6 @ 92.1 (1.6) in 1/8th