Hot Cars Act of 2021 This bill requires safety measures in passenger cars to reduce injuries or deaths from heatstroke and other harms to occupants of unattended vehicles. Specifically, the National Highway Traffic Safety Administration (NHTSA) must issue a motor vehicle safety standard that requires certain new passenger motor vehicles to include a system that detects and warns of the presence of an unattended occupant in the passenger compartment. The NHTSA must issue the final rule prescribing the safety standard within two years after the enactment of this bill, with full compliance with the standard required not later than two years after the rule is issued. After the standard is issued, the Department of Transportation must periodically contract with an independent third party to study retrofitting passenger motor vehicles with technologies and products that detect and warn when there is an unattended occupant in the passenger compartment. The study must identify relevant technologies and products to retrofit vehicles and provide recommendations about testing such technologies and products. Information about the performance of the technologies and products must be published on the NHTSA's website.