The unnamed family - which consists of two adults and three children - and their housekeeper had to be taken to hospital after displaying symptoms of the illness at their home in Little Aston, Staffordshire, in January last year.
They all suffered from vomiting and severe headaches and two of the group lost consciousness, leading to them requiring oxygen from paramedics at the scene.
An investigation conducted by the Health and Safety Executive (HSE) established that gas engineer Graham Slater - who carried out checks at the property in February 2009 and July 2010 - was not fully qualified to conduct such work.
Despite the fact Mr Slater is listed on the Gas Safe Register, he is not legally entitled to work with warm air heaters.
Wai-Kin Liu, inspector for the HSE, commented: "Every year, around 20 people die from carbon monoxide poisoning."
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