The expert commissioned can be challenged by one of the parties (article 234 of the new Code of Civil Procedure), the cause of challenge being assessed by the judges on the merits. It should be noted that article 341 of the new Code of Civil Procedure, to which article 234 refers and which provides for cases of challenge, does not necessarily exhaust the requirement of impartiality required of all judicial experts ( 1 st Civ ., April 28, 1998, Bull., I, n ° 155, p. 98; 2 nd Civ., December 5, 2002, Bull., II, n ° 275, p. 218). The request for challenge, which must be made before the start of operations or as soon as the cause for challenge is revealed, can no longer be made after the filing of the expert’s report (3 e Civ., June 20, 1979, Bull., III , n ° 139; 2 nd Civ., December 5, 2002, Bull., II, n ° 279, p. 220). The expert being an auxiliary of justice committed by the judge is not a third party to the dispute and, therefore, does not have the capacity to form a third party opposition to the decision of challenge of which he is the object (2 nd Civ ., June 24, 2004, Bull., II, n ° 314, p. 265).
It should also be noted that the decisions ruling on a request for a change of expert are not subject to the provisions of article 170 of the new Code of Civil Procedure relating to the execution of investigative measures (2 nd Civ. , October 18, 2001, Bull., II, n ° 158, p. 107). The decision by which a court rejects a request for replacement of an expert and the recusation of the latter puts an end to an incidental proceeding, independent of the main proceedings which gave rise to it, so that it is likely to appeal to the court of appeal, the decision of which is itself open to appeal (2 nd Civ., 23 June 2005, appeal n ° 03-16.627).