Tesla will repair over 2m vehicles, its biggest recall, after the US auto safety regulator determined the driver assistance system Autopilot doesn’t do enough to guard against misuse.

Bloomberg said the move was the result of a years long defect investigation which the National Highway Traffic Safety Administration would keep open to monitor the effect of Tesla’s fixes.

A NHTSA spokesperson told Bloomberg the probe found Tesla’s means for keeping drivers engaged were inadequate.

According to Bloomberg, Tesla said in its recall report it expected to start deploying an over the air software update to incorporate additional controls and alerts on or shortly after 12 December.

The recall is the second this year involving Tesla’s automated driving systems which have come under escalating scrutiny after hundreds of crashes, some of which resulted in fatalities.

Both Autopilot and the beta features Tesla markets as Full Self Driving require a fully attentive driver to keep their hands on the wheel.

Tesla has marketed higher level functionality it calls FSD Beta since late 2016. That suite of features was recalled in February, after NHTSA raised concerns about Teslas that were using the system traveling in unlawful or unpredictable ways, including exceeding speed limits and not coming to complete stops, Bloomberg said.

The two NHTSA ongoing defect probes were precipitated by Teslas crashing into first responder vehicles and suddenly braking on highways.