Paul Fromet, of Cordell Close, Cheshunt and Michael Wright of Lodge Crescent, Waltham Cross, were asked to fit a boiler in a house in Hargrave Road, Chevington, Suffolk that was due to be rented out by the landlord.
Bury St Edmunds Magistrates' Court heard that a complaint had been made about the installation to Gas Safe which was then passed to HSE and an investigation followed. Neither plumber was registered with Gas Safe, the compulsory registration scheme for gas engineers.
The boiler had not been adjusted to run on Liquefied Petroleum Gas (LPG), which supplied the property. As a result of this failure, tests on the boiler during operation showed that it was producing exceptionally high levels of carbon monoxide - more than 200 times the manufacturers' specification.
Both men admitted breaching Regulation 3(3) of the Gas Safety (Installation & Use) Regulations 1998. Both Mr Fromet and Mr Wright received a two-year conditional discharge and were ordered to pay costs of £250 each.
HSE inspector Edward Crick, said: "It is illegal for an unregistered person to carry out work on a gas appliance. When unqualified workers try to bypass the law in this way they are not only putting themselves at risk of prosecution and a large fine, they are also putting their customers' lives at risk.
"Working with gas appliances is difficult, specialised and potentially very dangerous. Only qualified and registered engineers should attempt it. HSE will not hesitate to prosecute those who break the law in this way."