On the 2002 Mercury Mountaineer, the Air Bag Code #42 is a fairly straight forward code pointing to a fault with the front Impact Sensor. The front impact severity sensor is mounted under the lower radiator support. Carefully inspect the electrical connector and as far up the harness as you can looking for any loose, broken or damaged wires. Make sure there is no excess build of debris around the sensor or connector. If the visual inspection checks ok, most commonly, replacing the sensor will solve the problem.
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