Before taking any other steps to eliminate the fishy smell in a car, first make sure that it is not coming from an antifreeze leak. This can be a serious problem and should be checked by a mechanic as soon as possible. If the odor is because of some other source, there are many at-home methods to try before going to a cleaning professional.
According to CarsDirect, the first step is to thoroughly vacuum the car's interior, making sure to access hard-to-reach places where smelly debris might be. If vacuuming does not remove the smell, use a portable steam cleaner on the carpet and fabric upholstery. Unpleasant car smells can come from air conditioning or heating ducts, so spraying a deodorizer through the car's intake valve can be helpful.
Sprinkling baking soda on the carpet, rubbing it into problem areas, and vacuuming it a few hours later may eliminate odors. Charcoal is often used to absorb smells in air and water filters. Placing a few chunks of charcoal in the car for several days may assist in removing persistent odors. Putting cotton balls that have been treated with a pleasant natural aroma like vanilla, mint or citrus can help mask the smell. Dryer softener sheets can serve the same purpose.
Consumer Reports and CarsDirect explain that some unpleasant car smells are caused by mechanical problems that may be serious. If the odor is that of gas or rotten eggs, it is best to take the car to a mechanic as soon as possible. Some odors, however, may persist after all at-home attempts to remove them. In that case, a consult a cleaning professional.