It will examine if the gas and electricity firm broke rules relating to customer information and contact.
That will include whether customers were treated “fairly” during call handling and complaint processes, whether indebted customers were given sufficient notice that Iresa was taking a debt repayment out of their account or established their ability to pay the debt.
The investigation will also look into whether the company enabled customers to switch suppliers on request in line with Ofgem’s rules and refunded customers’ money in a “prompt and timely” manner.
The regulator said the opening of the investigation does not imply it has made any findings about non-compliance by Iresa.
The probe follows concerns raised relating to a “high level of complaints” against Iresa, including through Citizens Advice and Ofgem’s consumer affairs team.
A spokesperson from the energy company said: “Iresa is currently looking at different issues raised by Ofgem. We continue to address these issues with aim of improving our customers experience.”