Staff from the charity used a “grasper” to remove the fox, and it later emerged the creature had triggered the warning light by chewing through wires in the engine.
After the animal was rescued, RSPCA inspector Herchy Boal said being trapped in a hot engine could have ended “very differently” for the fox.
Ms Blower said the team at the hospital had never seen a rescue like it.
“She was very timid when she first came in, she had obviously gone through quite a lot to get here,” said Ms Blower.
“We did a full assessment and got her onto some medication.”
The RSPCA added it hoped to release the fox back into the wild next month.