In a statement Tuesday, NASCAR said the infraction occurred Saturday before the Avenger 500 auto race at the Darlington (S.C.) Raceway.
Earnhardt's No. 8 DEI Chevrolet was found to have illegal brackets used to mount the rear wing on the Nextel Cup Series' new Car of Tomorrow.
Crew chief Tony Eury was fined $100,000, suspended for six races and put on probation until Dec. 31.
It matches NASCAR's biggest penalty.
"It was a bad deal," Eury said. "I was definitely not trying to get away with nothing, so you can take it for what it is. It was a part that got put on there that shouldn't have."
The penalty dropped Earnhardt from 12th place to 14th in the latest Nextel Cup points standings.
He announced last week he is leaving DEI at season's end.