SVT Nyheter Norrbotten reported on the incident on 17 September.
The ambulance was parked in a garage at Älvsbyn's health center when a gas explosion was triggered and the vehicle started to burn.
No person was injured but the incident was reported to the Swedish Work Environment Authority and now the investigation is complete.
"A probable reason was that there was dirt (grease) inside the gas system that ignited due to the frictional heat that arose when the ambulance personnel opened the gas cylinder and the oxygen flowed out at high speed inside the pressure regulator," the investigation writes.
Installed inappropriate gasket
It had previously been established that the pressure regulator was damaged.
The supplier repaired it and judged after functional inspection that it was approved.
"Medical technicians who handled the case did not have knowledge of the product's structure and installed an inappropriate O-ring where it was not needed."
The Norrbotten region is now taking several measures to prevent something similar from happening again.
Among other things, personnel will be trained in medical gases and handling of medical gas cylinders, including those connected to pressure regulators.