[3][4] After the 2002 season concluded, the FIA Formula One Commission met in London to decide on regulation changes.

Dennis commented that despite the lack of technical changes, the FIA should "leave the regulations alone and we will catch up and hopefully surpass the performance of Ferrari.

The latter acted in a similar fashion to a diffuser, creating a low-pressure area below the wing, sucking the nose and the front end of the car to the ground.

In order to resolve it Newey called for a chassis redesign, an almost insurmountable task in the middle of a season.

"[15] Changes to the points system for the 2003 season meant that there was greater emphasis on reliability, something with which Mercedes-Benz was struggling since the ban of beryllium.

[22] At the 2003 San Marino Grand Prix, Ron Dennis was asked on when the MP4-18 would debut, answering cryptically, "No sooner than Canada and no later than Silverstone.

"[9] The effects of the radical design changes were immediately clear at the MP4-18's test debut at the Circuit Paul Ricard.

Straight away, the car faced overheating problems due to the small area within the sidepods (in which the radiators are situated) that did not allow for the decompression of heated gas or air.

[23] At the Barcelona tests in June, Kimi Räikkönen did not fare well either, shunting at the final turn, bruising his knee[24] and writing that chassis off.

[25] Wurz successfully tested the MP4-18 for 330 km (205 mi) in Austria; however, when he came back into the pits it was determined that the internal temperature of the gearbox had reached 120 °C (248 °F), causing delamination.

[26] However, the Mercedes-Benz engine still remained problematic, with McLaren encountering reliability issues until the introduction of the MP4-19B upgrade at the French Grand Prix.

Following a string of poor performances, Newey eventually prevailed, introducing a new chassis with the MP4-19B, giving the team their only win of the season at the 2004 Belgian Grand Prix.

Newey, less angered so by the performance of the car, felt that the vote had essentially vetoed his position as technical director.

It later came to light that following Newey's Jaguar F1 tussle a few years earlier that Ron Dennis engineered a change in McLaren's management structure to ensure that such a key figure leaving could not devastate the team again.

Newey's dissatisfaction eventually resulted him announcing in November 2005 that he was joining Red Bull Racing for the 2006 season.

[27][28] Motorsport writer Edd Straw opined in an article in 2023 that the car was too ambitious for the time, and that McLaren-Mercedes' and Newey's obsession in perfecting the MP4-18's design for so long and prioritising its development over the MP4/17D's for much of the season was one of factors that cost Kimi Räikkönen the 2003 Drivers' Championship.