Jimmie Johnson's No. 48 team avoided penalties but did receive a written warning from NASCAR on Tuesday for altering the side skirts on their car during last Saturday night's NASCAR Sprint All-Star Race at Charlotte Motor Speedway.

Prior to the start of the 2015 season, NASCAR made a rule modification that prohibited Sprint Cup Series teams from flaring the side skirts -- the sheet metal ahead of the rear tires on the car -- during pit stops in a race. It had the potential to improve air flow over the rear of the car, which would help increase speeds. The practice became widespread in Sprint Cup during the latter half of the 2014 season.

NASCAR will issue a warning to any team in their first offense of altering the side skirts and then assess penalties if there are repeat offenses.

Johnson, the six-time Sprint Cup champion, finished 15th out of the 20 drivers who participated in the all-star race, which is a non-points event.