Custer, who has two NASCAR Xfinity wins this year, has been docked 20 driver and owner points as a result of the L1-level penalty.
Additionally, both driver and team have been assessed with the loss of five playoff points, dropping them from 15 to 10.
Crew chief Jonathan Toney has been fined $25,000 as well.
Custer competed fulltime in the Cup Series from 2020 to 2022, winning once at Kentucky. He has now returned to the Xfinity Series, where he has 12 career victories. He currently sits fourth in the regular season standings.
NASCAR says the No. 00 Ford violated Sections 14.4.3 C & H: Splitter of the NASCAR Rule Book, stating in the penalty report: "Except as specified, splitters must be used exactly as supplied from the manufacturer."
Stewart-Haas Racing will not appeal. No other penalties were issued following the Michigan race weekend.
Joey Logano's No. 22 Team Penske Ford and Kyle Larson's No. 5 Hendrick Motorsports Chevrolet were taken back to the R&D Center for further inspection following the Cup race, but no issues were found.
