Marcus Ericsson says his wheel "basically just fell off" before his problem towards the end of the sixth day of winter testing in Barcelona.
Ericsson triggered red flags late in the afternoon when he stopped on the gravel at Turn 4, ending a delayed first day for the Swede behind the wheel of Sauber's 2016 challenger.
Asked to explain what happened, Ericsson said: "We're not sure 100 percent yet, they are investigating. Basically it just fell off when I went into the corner .. It was just the wheel [not suspension].
"No, no warning. It was my third lap, I think.
The Swedish driver only had 55 laps to his name before that issue, which he put down to longer issues from earlier in the day.
"First of all it was an engine change overnight which took a bit longer than expected. Then when we got out on track there were still some issues related to the power unit that held us back and made us do some outlaps. It's frustrating but that's how it goes sometimes in the first couple of laps in a new car."
Ericsson will return to the car on Friday, the final day of testing before the Australian Grand Prix.