A fire at Threeways Garage in Abergele at the weekend was caused by an electrical fault which ignited a cupboard, according to North Wales Fire & Rescue Service.
Firefighters were on site for four hours on Sunday night tackling the incident, which has caused fire damage to around a third of the showroom plus extensive smoke damage, the service said.
Threeways has franchises for the Fiat, Hyundai, MG and Suzuki car brands.
Emergency services closed the road at the dealership, which adjoins a Shell fuel forecourt.
Nobody was injured in the incident.