Cars and water don't mix. Hundreds of thousands of people learned that the hard way after Superstorm Sandy, and more are learning it after the deadly floods that recently struck Texas. Salt is more damaging to cars than fresh water because of its corrosive nature, but going under water of any kind is bad for a vehicle. "Flood water is a corrosive and abrasive mixture of water and dirt that works its way into every seam and crevice of a vehicle," warns AAA.

So what should you do if your car's been in a flood? That depends. 

Related: What To Do After a Flood

How flooded is too flooded?

Try to determine how high the water reached. It is possible that the level wasn't high enough to do any significant damage.

The bad news: Water that rises above the floor is typically enough for a car to be considered totaled, according to CarTalk.com. Once water hits the engine compartment or washes into the interior of the vehicle, too many key parts will likely be damaged to safely operate the car. According to AAA, "The engine, transmission and drive train, along with the fuel, brake and power steering systems are most vulnerable to flood damage." AAA notes that unless all debris that enters the car's components along with the water is removed, you can expect parts to fail.

While even the most severe damage will often not be plain to the untrained eye, a water-logged vehicle, even after being dried out, will not be safe to operate properly without significant repairs. Even then, mechanics do not recommend fixing flooded cars because of how extensive the damage is likely to be and the inability to completely remedy the issues.

"Any vehicle can be repaired if you want to spend enough money to fix it," notes Davis Repair, a Denver area auto shop. But fixing it might not make financial sense. According to the site, as a general rule, if the water has risen above the bottom edge of the car doors and soaked the interior, it will probably cost more to fix the car than it's worth.

It is possible to salvage an engine computer, sensors, and other such devices, but according to AAA, "unless they are thoroughly cleaned and dried, inside and out, problems caused by corrosion and oxidation may occur weeks or even months after the flooding." Because of how difficult it is to access certain areas of a car, that isn't always possible.

Related: 5 Ways to Avoid Auto Body Shop Ripoffs

What to do

Many mechanics, AAA and the National Auto Dealers Association advise that you shouldn't start up a vehicle that has been flooded. Instead, call your insurance company. Unlike homeowners, who have to buy separate flooding coverage, most motor vehicle owners are covered for such damage through the comprehensive coverage portion of their auto insurance policies. Unlike liability insurance, comprehensive coverage isn't required, so some people don't have it. But the Insurance Information Institute says about three-quarters of all policies include it.

Take pictures that show how far the water reached, or of the actual flooding if you can. If you can't take photos, make note of how high the water reached.

The insurance company will determine if repairing the vehicle will cost more than it's worth before designating it as a total loss. Often, AAA says, the amount of work necessary to repair a flood-damaged car will exceed the value of the vehicle.

Be wary of car sales after floods

Because so many cars get totaled after floods but can easily pass visual inspection, it's common for water-damaged cars to be sold by less-than-honest dealers. It's one thing for someone to buy such a car knowing that it has been through such an ordeal and quite another for that detail to be concealed.

Usually, when a car is totaled, that will be recorded and linked to the car's Vehicle Identification Number (VIN). Using a paid service like CARFAX prior to purchase should reveal the car is considered "salvage" or has some other designation that indicates that it suffered damage. In addition, the National Insurance Crime Bureau offers a free service that allows consumers to check to see if a car being sold is stolen or considered "salvage."

While there are some tell-tale signs, such as brittle wiring or rust in unexposed areas, it is best to have a vehicle examined by a mechanic before you purchase it because many of the problems caused by floods won't be obvious to an untrained eye. Simply put, you don't want to buy a flood-damaged car, even at what seems to be a great price. A flood-damaged vehicle that might drive OK initially still should not be considered reliable if it has flood-damaged parts, which are far more likely to fail prematurely.

Related: How to Buy a Safe Used Car

Mitch Lipka is a consumer columnist and product safety expert. He was the 2011 recipient of the "Kids Best Friend Award" from Kids In Danger for his commitment to reporting on children’s product safety.