Specifically, the investigation centers around two aspects of how the companies do business, according to Bloomberg. Investigators want to know whether the original equipment component makers are able to sell spare parts only to automaker-authorized dealers or if they are also available to independent shops. There is also the issue of whether the price markup on replacement pieces is too high. The tight controls could be partially explained by China's reputation for producing counterfeit parts.
Evidently, the investigators haven't checked parts prices at car dealers elsewhere in the world. At least in the US, paying more at the dealer for factory components just goes along with owning a vehicle. If evidence of price fixing is found, the companies could face fines the equivalent of millions of dollars, according to Bloomberg.