ROKiT Williams Racing's technical director Paddy Lowe is set to take a "leave of absence" from the team ahead of the 2019 Formula 1 season for personal reasons.
The auto was also the slowest on track when it did appear, adding to the woes of a team who finished last overall in 2018 and have not won a race since 2012.
Pre-season testing for the Grove based team has been tough the late debut of the FW42.
The auto eventually ran for the first time on the third day of the eight-day pre-season test.
The season begins with the Australian Grand Prix on March 17.
Williams director Lowe takes 'leave of absence'
As well as his responsibilities as technical chief, Lowe acquired a shareholding in Williams when he joined.
But on Wednesday evening a short Williams statement confirmed that "Paddy is taking a leave of absence from the business for personal reasons".
Williams completed the least mileage across the course of the two tests and its vehicle was comfortably the slowest, with both Kubica and Russell finishing at the foot of the timesheets.
Last year's auto was the first to be designed and built under Lowe's leadership, following his high-profile and expensive move to Williams from Mercedes in early 2017.
Deputy team principal Claire Williams last week denied claims the auto was not completed on time due to delays with external suppliers or financial problems.