Seven-year-old Zane Gbangbola died on February 7 2014 after the outside of his home near the Thames was engulfed in flood water.
His parents believed the water was contaminated with toxic hydrogen cyanide from a nearby lake built on a former landfill site.
But Woking Coroner's Court heard evidence that the boy's parents hired a petrol pump to clear the flood water in their basement, which was used for up to six hours on the day the boy died and pumped deadly fumes into Zane's upstairs bedroom while he slept.
He was discovered unconscious at around 3.30am the following day by his mother, Nicole Lawler, and pronounced dead an hour later in hospital.
Ms Lawler and Zane's father Kye Gbangbola, of Thameside in Chertsey, Surrey, have spent the last two-and-a-half years campaigning for further investigation after disputing post-mortem examination results that said Zane's death was because of carbon monoxide poisoning.
Coroner Richard Travers concluded: "I find the cause of death was carbon monoxide toxicity, from fumes generated by a petrol pump used by his family to clear the house of flood water."
Mr Gbangbola had been working in another bedroom upstairs from 7pm on the day his son died.
The father was found slumped on his bed in a half-seated position when paramedics were called to the scene.
He was paralysed and wheelchair-bound as a result of the incident, while his wife - believed to have been "snoozing" that evening - made a full recovery.
The court heard Ms Lawler only rented the Honda petrol trash pump from Surrey Hire Services the day before Zane died.
But the couple's testimony in court that the pump was only "tested" for a few minutes and that it was switched off before lunchtime was rejected by the coroner - they had previously said it had been used until the early evening, the court heard.
Mr Travers said: "I have no hesitation that Ms Lawler and Mr Gbangbola did not want the pump to be working when Zane went to bed.
"But I cannot accept their accounts that on February 7 it (the pump) was used for no more than 20 minutes and not after lunchtime."
Surrey Police said no criminal charges would be brought over the boy's death.
Speaking outside the court, Ms Lawler said the inquest was "legally and evidentially deficient".
She said the family will now request an independent, panel-led inquiry similar to the Hillsbrough inquests providing a "thorough and open" examination.
