The FIA is looking into what happened in Graz, Austria, between the final Bahrain test and the season opening Australian grand prix.
A whistleblower last week circulated letters to the governing body and some rival teams about an alleged illegal six-day test on Austrian automotive company AVL's 'rolling road' facility.
Engine supplier Renault's Remi Taffin is quoted by APA news agency: "We just had to work with our partner AVL, because our own facilities were at full capacity."
The French marque argues that the 'test' was in fact just perfectly legal engine bench testing, amid reports the Toro Rosso car was not fitted with front or rear wings.
Nonetheless, an FIA spokesperson said: "We will look into the matter carefully and then answer any questions."