There can be many reasons that there is a bad smell in the car. The most common reasons are that someone has smoked in the cabin, that animals have been in the car or that something has been spilled on the upholstery or carpeting.
With odour removal, the cabin is filled with a gas generated by vaporizing various chemicals. Which chemical to be used is determined by which source has caused the bad smell. The gas is allowed to operate for a period of time after which the cabin is aired out. The treatment is repeated if necessary, if not all odors have disappeared.
The reason why it smells bad – the source – must of course be found and then removed. The odor removal can sometimes need to be combined with an upholstery cleaning or even require dismantling parts of the interior to get to grips with the problem.