The baby's mother, Lacey Te Whetu, told her she'd last seen him breathing two hours earlier when she'd put him down for that sleep. He'd recently had the flu.
When English first saw the baby an older woman and man were attempting CPR, and she later established the woman was Parangi.
She and other paramedics were unable to bring him back to life.
English said when the baby's mother was told her son was dead she cried out "come back to me my baby".
His father Shane Christopher Neil didn't appear to show any outward emotion. The baby's mother pointed out a "fresh" bruise on his stomach after he was pronounced dead.
English then noticed other, older bruises including one inside the infant's elbow.
Parangi's next door neighbour, Sharon Biddle, tearfully told the jury Te Whetu was performing CPR on him while on the phone to the ambulance, "trying to bring her baby back".
Isaiah's parents pleaded guilty last month to their son's manslaughter.
Biddle said earlier in the day she saw Neil sitting on the back doorstep and a station wagon parked in the Te Whetus' driveway with the windows up.
The crown alleges baby Isaiah spent at least three hours in the car while his grandmother and mother binged on synthetic cannabis.
To prosecutor Anna Pollett she said the day was hot and although it would have been shaded in the morning come the afternoon the sun would have been blazing down on it.
The court later heard Parangi claimed in a statement to police she had seen her grandson asleep in the family car with its windows down and sunroof open.
She said she'd had her eye on Isaiah while she was hanging out washing after she and her daughter returned from a trip to Kawerau with the baby, saying they left him in the station wagon because he was sleeping.
The trial continues.
