The National Highway Traffic Safety Administration (NHTSA) is investigating allegations that front air bag sensors may fail because of corrosion in several DaimlerChrysler minivans, causing the air bags to improperly inflate during a crash or not to deploy at all.

The investigation involves about 805,000 2005 and 2006 DaimlerChrysler Dodge Caravan, Grand Caravan and Chrysler Town and Country minivans.

NHTSA reports it has stepped up a preliminary evaluation of the problem to the status of an "engineering analysis," a move that often precedes a safety recall.

The preliminary evaluation determined that the front air bag sensors might corrode as a result of water seeping in to the sensor assembly.

The Caravan and Grand Caravan minivans have an "advanced frontal air bag system," according to NHTSA with "two front crash sensors mounted on the frame rails behind the front bumper and a main crash sensor inside the air bag control module mounted in the passenger compartment."

DaimlerChrysler reported to NHTSA that the sensors are designed to "optimize detection of frontal offset and angular crashes" and the air bag system offers different levels of inflation based on the input from the various sensors.

"The corroded sensor can set off a fault code, illuminate the air bag warning light and disable the air bag," according to NHTSA.

"The failure of one or both front crash sensors can potentially result in no deployment or late deployment of the front air bags. The failure can cause the non-deployment of the drivers side air bag which produces higher inflation levels," according to the NHTSA Web site.

The NHTSA report states that "it should be noted that occupants of vehicles involved in a crash may not be aware of late or improperly reduced level of air bag inflation" because of the nature of the system in the vehicles.

The agency is also investigating 358,455 Dodge Durango SUVs from 2004 to 2006 model years and 2005-2006 Dodge Dakota pickup trucks after receiving complaints about loose steering wheel and shaft coupling bolts.