In 1989 Lehto tested for Ferrari before making his Formula One debut for the Onyx team as a late-season replacement for Bertrand Gachot. Though he failed to prequalify for his first race at Estoril he impressed with his speed in the tough sessions and made his first start in the following meeting. In the wet season finale at Adelaide he ran as high as 5th before retiring with waterlogged electrics. Over the summer Onyx were sold to Swiss racer turned businessman Peter Monteverdi. Lehto, marked by many as a star of the future, was paired with Gregor Foitek but financial difficulties hampered his season, leading to the team's withdrawal after the Hungarian Grand Prix (one of five events the Finn failed to qualify for – though he had been hindered by the team's poor preparation, including a driveshaft being fitted the wrong way around for several meetings, and favouritism towards Foitek, whose father was involved with the buyout).