Fiat Chrysler under-reported a "significant" amount of deaths, injuries and legal claims that it was obliged to reveal to regulators, the US's car safety watchdog said.
The National Highway Traffic Safety Administration made the announcement after Fiat Chrysler said it had discovered "deficiencies" in its system for reporting faults under the Tread Act, which governs what information carmakers have to give their regulators, while investigating discrepancies in figures.
The NHTSA regards early warning reporting as vital to its efforts to sift through the 33,000 annual deaths on the US roads to identify which are caused by serious design flaws with vehicles.
The news comes as NHTSA steps up its enforcement of the safety rules surrounding vehicle faults following controversy over General Motors' botched recall of vehicles fitted with faulty ignition switches.
Fiat Chrysler in particular has suffered a series of run-ins with the regulator, which took the unusual step of holding a public hearing in July to investigate the company's handling of a series of potentially dangerous faults.
The company — formed when Italy's Fiat took over America's Chrysler following its 2009 government-managed bankruptcy — also engaged in a protracted dispute with the regulator in 2013 over whether to recall 2.7 million older Jeep models that the regulator said were prone to exploding in rear-end collisions.