bracket to the top of the battery coolant reservoir to help prevent potential coolant overfill. The additional side safety structural pieces have a total weight of , and their function is to spread the load of a severe side impact away from the battery pack, reducing the possibility of intrusion into the pack. During December 2011, GM conducted four crash tests of Volts with the reinforced steel and upgraded cooling system, resulting in no intrusion to the battery and no coolant leakage. On December 22, 2011, the NHTSA also subjected a modified Volt to the same test that led to the original fire, with no signs of the damage that is believed to have been the cause. The NHTSA said "the preliminary results of the crash test indicate the remedy proposed by General Motors today should address the issue of battery intrusion" though its investigation remained open. General Motors declined to say how much the modifications would cost. All 12,400 Chevrolet Volts produced until December 2011, including all Amperas in stock at European dealerships, were scheduled to receive the safety enhancements. Since production was halted during the holidays, the enhancements were in place when production resumed in early 2012. Sales continued, and dealers modified the Volts they had in stock. General Motors sent a letter to Volt owners indicating that they could schedule the service appointment to protect their batteries beginning in the last week of March 2012. General Motors also decided to replace the 120-volt charging cords in most of the nearly 10,000 Volts sold since late 2010. The new cords were enhanced to add durability, and some of the chargers built after February 5 have the new cords. NHTSA findings On January 20, 2012, the National Highway Traffic Safety Administration closed the Volt's safety defect investigation related to post-crash fire risk. The agency concluded that "no discernible defect trend exists" and also found that the modifications recently developed by General Motors are sufficient to reduce the potential for battery intrusion resulting from side impacts. The NHTSA also said that "based on the available data, NHTSA does not believe that Chevy Volts or other electric vehicles pose a greater risk of fire than gasoline-powered vehicles." The agency also announced it has developed interim guidance to increase awareness and identify appropriate safety measures regarding electric vehicles for the emergency response community, law enforcement officers, tow truck operators, storage facilities and