Diesel buses replaced electric buses in York due to charging infrastructure works being delayed at First’s James Street depot.
Four diesel buses from Leeds were being used out of a fleet of 90 as of 19 February.
Problems had arisen due to an issue with the level of power supply required to charge them all overnight.
Northern Powergrid engineers are estimated to start work to solve the issue in April at the earliest.
The Leeds buses are all Euro VI models and compliant with York’s ‘Clean Air Zone’ requirements.
Tom Donnelly, operations manager of First York, said: “Due to planning and the excellent efforts across our depot team we maintained services with no inconvenience to journeys.
“The disruption to the power output experienced at our depot has been identified and work is well underway to rectify this.
“We retained one or two of the vehicles brought in last week from nearby depots to support service delivery while the power system stabilised.
“All the buses from other areas have now been returned.
“We appreciate the patience and understanding of customers.”
‘Far from ideal’
The council’s transport executive, Coun Pete Kilbane, said: “It is disappointing that problems with the Northern Powergrid’s electricity supply to the First Depot have resulted in a very small number of diesel buses being used to cover the routes.
“Whilst this is far from ideal we think that residents appreciate that a diesel bus is better than no bus at all.
“We will continue to work with the bus companies to bring stability to the bus network and improve reliability.
“One bus failing to turn up is one too many.”
Former Green Party transport executive, Coun Andy D’Agorne said: “I understand the problems that First has experienced and appreciate the efforts made to maintain bus services at short notice.
“However it does highlight the need for backup overnight charging arrangements maybe at park and ride sites.”