The beginning of the 2003 season saw Hornish struggle in comparison with his previous two years at Panther. At the Indianapolis 500, he qualified in 18th place; in the race, he retired with a blown engine after completing 195 of the 200 laps. Hornish finished the race in 15th place, dropping to a tie with Buddy Rice for 12th place in points. At the following race at Pikes Peak, he started in 14th place and finished fifth (his first lead-lap finish of the season). Around this time, Panther Racing and several other teams using the Chevrolet engine built by General Motors (the parent company of Chevrolet) switched to a new engine by Cosworth which was badged as a Chevrolet engine. After the change Hornish's results began improving; he started fourth and finished second, after leading for 126 of 200 laps, at Michigan International Speedway. Two races later, at Kentucky Speedway, he qualified for the pole position and led for 181 of 200 laps to win. Hornish then won two more consecutive races, at Chicagoland and at California Speedway. He won the former race by 0.01 second over Scott Dixon and Bryan Herta, the closest top-three finish in series history. Going into the season-ending Chevy 500 at Texas Motor Speedway, Hornish was mathematically eligible win the championship; however, his car had a spray problem after 176 of 195 laps. He finished 17th, and was fifth in the final point standings with 461 points.