Detective Inspector Robert Kirby said: "A unique set of events came together to allow this tragic incident to happen. A gap in the exhaust system where the exhaust meets the engine allowed fumes to escape.
"Compounded by the fact the car had had its catalytic converter removed (which normally dissipates harmful emissions). Vents underneath the windscreen wipers then in-took the emissions from the gaps in the bonnet."
Residents told how they had heard the engine of Putt's car running at around 4.30am on December 5 in Chelmsford. The alarm was raised at 10.30am.
It is believed the cold weather also allowed the fumes to congregate.
Ford assisted Essex Police in tests and carried out analysis on Putt's car and a factory issue model. This was when the dangerous level of toxic emissions was discovered.
Both deaths were declared to be accidents by coroner Caroline Beasley-Murray at Chelmsford Coroner's Court today.
She read post-mortem reports that each of them died of inhalation of exhaust fumes including carbon monoxide.
Following her summary, she said: "I hope the families will be able to think of the happy memories".
Speaking outside court on behalf of the family and Essex Police, Detective Inspector Kirby said: "This was a tragic accident which led to the deaths of Nikki and Tom.
"Tom was not inexperienced. He had been an apprentice with Ford for over a year and had done work on his car previously. We would like to encourage people to please consider if modifications are worth the potential consequences."