A gas-installer was fined after an investigation into his work revealed that he had poisoned an elderly couple.
Dermot Healy was prosecuted by the Health and Safety Executive (HSE) for carrying out gas work while not being Corgi registered and for not holding Competence Certificates which are needed for specific types of gas work.
Mr Healy performed work despite not having passed the necessary gas training courses or commercial gas courses which would have allowed him to do so.
An elderly couple from Harrow were treated for carbon monoxide poisoning two weeks after Mr Healy had carried out work on their gas-fired Lennox warm-air unit
Mr Healy serviced and was asked to re-comission the unit in 2007, which had been in operation since 1961 and reportedly condemned by previous gas engineers, but made no recommendations.
In 2002, contractors replaced the door to the boiler cupboard door, which did not have any air-vents and Dermot Healy soon installed a plastic vent cover.
A British Gas investigation discovered that the air unit cupboard had inadequate ventilation and the flue was also of an insufficient length.
This lack of ventilation restricted the release of exhaust gases to the outside while negative pressure inside the cupboard resulted in gases being drawn down into ducts in the unit.
A fortnight after Mr Healy's work, the elderly couple were taken to hospital after feeling lightheaded and dizzy before falling unconscious.
The City of London Magistrates' Court fined Mr Healy 500 and ordered him to pay costs of 1,500 after he pleaded guilty to breaching Regulation 3(3) of the Gas Safety (Installation & Use) Regulations 1998.
Mr Healy was also prosecuted under Section 3(2) of the Health and Safety at Work etc Act 1974.
Further HSE inquires discovered that Dermot Healy serviced a boiler in 2006 despite not being CORGI registered and not having the required Competence Certificate to carry out work on these units .
HSE Inspector Charles Linfoot said: "The couple in this instance were lucky, but the outcome could so easily have been a tragic one."
"This work was fairly minimal, and was done on an informal basis. However, he acted illegally in that he was an un-registered gas engineer and carrying out work he was not qualified to do."
Gas Safe Register replaced CORGI as the gas registration body in Great Britain and Isle of Man in April 2009 and Northern Ireland and Guernsey in April 2010.
Only engineers who are Gas Safe registered can carry out gas work in the UK.