Peter Tongue wrongly believed he had repaired the appliance
Two friends died of carbon monoxide poisoning after a gas fitter allegedly failed to fix a central heating boiler, Cardiff Crown Court has heard.
Peter Tongue, 60, was paid £60 to fix widow Margaret Powell's heater but left it in a "lethal" state.
Twelve days later Mrs Powell, 72, and her friend Gwyn Morgan, 74, were found dead in the bungalow in Brecon, Powys.
Mr Tongue denies two charges of manslaughter and seven charges under the Health and Safety at Work Act.
The court heard that Mr Tongue, of Llanspyddid, near Brecon, went to Mrs Powell's home on 4 December 2006 to carry out repairs to the boiler.
Prosecutor Peter Davies said when the appliance was examined part of it was found to be congested with soot.
"As a consequence, it was giving out carbon monoxide and giving it out in lethal quantities," Mr Davies said.
He said the prosecution alleged Mr Tongue was responsible for the deaths "because when he carried out work for Mrs Powell he failed to carry out work on her central heating system properly".
He said: "It is alleged he carried out work on an appliance of a type he didn't understand... and took no steps to find out what he should do with that type of machine to do it safely and efficiently.
"Tongue is guilty of gross negligence - had he done his job properly the deaths would not have occurred."
Tests carried out on the boiler after it was cleaned of soot found it worked efficiently without producing carbon monoxide, said Mr Davies.
It was gross negligence on Mr Tongue's part that led to the friends' deaths, he added, as he owed them both a duty of care to carry out work to recognised standards, competence and efficiency and failed to do so.
The house where the bodies were found in Brecon.
The court heard that Mr Tongue worked in the gas industry all his life.
He was described as a "one-man band" servicing and repairing domestic heating appliances in Brecon.
However, although he had been with Corgi, his registration allowing him to work on ducted warm air systems had lapsed in 2003 - three years before the deaths of Mrs Powell and Mr Morgan.
The court also heard that when other ducted warm air heaters he had worked on were inspected, they were found to be in an immediately dangerous condition.
Mr Davies described grandmother Mrs Powell as an active woman who still worked part-time as a receptionist in an optician's.
Mr Morgan lived with his son in Cwmavon, Port Talbot, but regularly visited Mrs Powell, a widow he had been friends with for six years.
The court heard the pair were overcome by carbon monoxide gas as they sat together on the living room sofa.
When the couple were found, their dinner of jacket potatoes was still in the oven and an uncooked pizza was on the kitchen table.
The trial continues.