A public inquiry has been called in Bristol following the publication of a report detailing how First Bus maintains its vehicles.
First Bristol says it is working to solve the problem
The inquiry is scheduled for 12 July after spot checks by the Vehicle and Operator Services Agency.
In this instance, Commissioner Philip Brown has the power to revoke, suspend or amend First's licence in Bristol.
A spokeswoman for First said: "We are confident that we have taken robust action following these spot checks."
Meanwhile, First has introduced some fare increases across the city recently.
An adult single fare went up from 80p to £1.00, £1.30 to £1.50 and £1.80 to £2.00.
In a statement, the company said: "The difficult decision to increase fares was made as we search for a way to cover the increased costs of operation brought about in part by significant increases in fuel prices."