Donna Parangi told police that she had seen her grandson asleep in the family car with its windows down and sunroof open.
Parangi, 48, of Ruatoki, has pleaded not guilty to the manslaughter of Isaiah Neil in November 2015 by depriving him of the necessities of life and failing to take reasonable steps to protect him.
She said she'd had her eye on Isaiah while she was hanging out washing after she and her daughter returned from Kawerau with the 8-month-old, saying they left him in the car because he was sleeping.
In the High Court at Rotorua yesterday, paramedic Linda English said staff worked on the infant in the ambulance for 26 minutes before establishing he couldn't be revived.
English said when she first saw the infant on a bed in his family home she hadn't seen any signs of life but he was warm.