Shares of Tesla surged 10% to end near a record high on Wednesday, extending a two day rally after it was announced the electric car maker will join the S&P 500.

The California company's stock (TSLA) has jumped nearly 20% since S&P Dow Jones Indices announced late on Monday it would add Tesla to Wall Street's most watched benchmark as of Dec. 21, a change that will force index funds to buy around $50 billion of its stock.

A blockbuster quarterly report in July cleared a major hurdle for Tesla's potential inclusion in the S&P 500, leading to speculation that the company, now with a market capitalization over $450 billion, might be added to the index and spark a surge in demand for its shares.

Up about 500% in 2020, Tesla has become the most valuable auto company in the world, by far, despite production that is a fraction of rivals such as Toyota Motor, Volkswagen and General Motors.

Its stock tumbled 21% in one session on Sept. 8 after it was left out of a group companies being added to the S&P 500, underscoring how much many traders expected it to be added.

Meanwhile, the U.S. National Highway Traffic Safety Administration (NHTSA) said on Monday it was expanding a probe into nearly 159,000 Tesla Model S and Model X vehicles, upgrading it to an engineering analysis, a step required before it can seek to compel recalls.

The auto safety regulator had opened a preliminary evaluation in June over touchscreen failures. NHTSA said the failure can result in the loss of rear-camera image display when in reverse and reduced rear visibility when backing up, and can impact defogging ability, and audible chimes relating to driver assistance system Autopilot and turn signals.

Tesla did not immediately respond to a request for comment.

The probe now covers 2012-2018 model year Tesla Model S and 2016-2018 Model X vehicles. The preliminary investigation covered 63,000 Tesla Model S cars.

NHTSA said the failure does not affect vehicle-control systems.

The media control unit (MCU) uses an Nvidia Corp Tegra 3 processor, NHTSA said.

The flash devices have a finite lifespan based on the number of programs or erase cycles, NHTSA said.

Some complaints said failures could result in loss of charging ability and that other safety alerts could be impacted. One driver said he could not clear fogged windows because he could not change climate controls.

In total, NHTSA said it has reviewed 12,523 claims and complaints about the issue, which would impact roughly 8% of the vehicles under investigation.

Tesla said it has received 2,399 complaints and field reports, 7,777 warranty claims, and 4,746 non-warranty claims related to MCU replacements.

Many complaints said Tesla requires owners to pay to replace the unit once warranties expire.

NHTSA said the data showed "failure rates over 30% in certain build months and accelerating failure trends after 3 to 4 years-in-service." Tesla has implemented over-the-air updates "to mitigate the effects of MCU failure," NHTSA said, including changes to reduce memory usage of the subject memory card, improving storage management strategies, and changing the control logic for turn signal activation.