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.
Under the Tread Act, manufacturers are required to report to NHTSA within five days of the end of each month any claims that their vehicles have been responsible for crashes resulting in deaths or injuries of any severity.
The NHTSA said that it had warned Fiat Chrysler in July that it had found an apparent discrepancy in its early warning data.
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"FCA [Fiat Chrysler] has informed NHTSA that in investigating that discrepancy, it has found significant under-reported notices and claims of deaths, injuries and other information required as part of the early warning reporting system," Mark Rosekind, NHTSA's administrator, said.
Preliminary information suggested that the under-reporting was the result of "a number of problems" with Fiat Chrysler's systems for gathering and reporting early warning reporting data, Mr Rosekind added. "This represents a significant failure to meet a manufacturer's safety responsibilities."
Early warning reporting data will often include large numbers of incidents that, on closer examination, turn out not to reflect a vehicle fault or not to represent a systemic problem requiring a vehicle recall. However, at July's hearing, the NHTSA raised concerns about possible under-reporting of problems with vehicle transmissions and tyre faults at high speed.
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Fiat Chrysler has signed a consent agreement with the NHTSA over the earlier safety concerns that commits the company to closer monitoring of safety issues.
The company said that, because of "heightened scrutiny" under that order, it had identified "deficiencies" in its reporting.
"FCA US promptly notified NHTSA of these issues, and committed to a thorough investigation, to be followed by complete remediation," the company said.
Karl Brauer, an analyst for Kelley Blue Book, a car information site, said the announcement reflected the greater scrutiny facing automakers over their safety records. The NHTSA has considerably lifted its safety enforcement after it failed for years to detect problems with ignition switches in a series of General Motors compact cars linked to at least 124 deaths.