Difference between PE and EA
FYI - for those wondering, NHTSA's upgrade means that the investigation is moving on to the next stage. Generally, a Preliminary Evaluation is a look see to determine if a problem exists. At this stage, a manufacturer gets a chance to provide information and data (e.g., warranty return rates) and an explanation of what happened. Most times, manufacturers will also put forth their case of whether the alleged defect is safety related or not.
Under the National Traffic and Motor Vehicle Safety Act, NHTSA has the authority to order recalls for vehicles or equipment that fails to comply with Federal Motor Vehicle Safety Standards OR for defects that are "safety-related." The question of whether a defect is "safety-related" or not is often the subject of much debate and negotiation between NHTSA and the target manufacturer.
If NHTSA is not satisfied with the data and explanation provided by the manufacturer at the PE stage and believes that further inquiry is warranted, the Agency will "upgrade" the investigation to an Engineering Analysis. In this stage, NHTSA may actually obtain defective parts and check them - and they will further grill the manufacturer. An EA is the last step before an ordered recall. Unless the manufacturer has got a darned good explanation - i.e., better than the one they tried at the PE stage, a recall will result. Although recalls do not always occur after an EA, you can bet that Nissan's got some 'splainin' to do!