Technical delegate Jo Bauer said in a statement that the Frenchman's car did not comply with the technical regulations because its front floor flexed more than permitted when a load was applied at a given point.
The matter was referred to stewards, one of them former Formula One racer and Le Mans winner Allan McNish, who could strip Grosjean of his time.
The Frenchman finished third in the previous race in Germany and qualified behind Mercedes' Lewis Hamilton, on pole, and Red Bull's triple champion Sebastian Vettel.
The cars of Hamilton and Vettel were also tested and found to conform to the regulations.
Any sanction would likely lift Ferrari's title contender Fernando Alonso, who is second overall and 34 points behind Vettel, from fifth to fourth.
(Reporting by Alan Baldwin, editing by Stephen Wood)