Devices to help parents avoid accidentally leaving a child behind in a hot car can be unreliable if used alone, according to the National Highway Traffic Safety Administration.
Many of the devices connect to child restraints. They use sensors to determine when a child is left behind in a vehicle, then alert a parent.
According to the study, some of the devices were rendered ineffective by liquids or cellphone interference, others when children moved.
Researchers fear the devices may give parents a false sense of security.
Heatstroke is the leading cause of non-crash, vehicle-related deaths for children under age 14, according to the NHTSA.
From 1998 to 2009, 494 children died when left unattended in hot cars. Heat stroke killed 33 children in 2011, 49 in 2010.
According to the NTHSA, parents should make it a habit to leave a cellphone, purse, briefcase or something else they need when they get their of the car, in the backseat with their child, .