Economy Energy is being investigated by Ofgem amid fears its marketing and telesales practices could have broken the rules.
The small Coventry-based firm mainly supplies gas and electricity to households with prepayment meters.
Regulator Ofgem said its probe would focus on "concerns" about how the supplier recruits new customers as its behavour may have breached licence conditions to communicate "transparently and accurately" with customers.
This isn't the first time the firm has been investigated - in November 2015 the regulator fined it £250,000 for mis-selling. In 2014 it was warned over poor customer service.
An Economy Energy spokesperson said: "We are co-operating fully with Ofgem and customer satisfaction remains our utmost priority at all times."
If you think you've been incorrectly billed or mis-sold an energy contract, check out our Energy Complaints guide for help on getting it sorted.