P0078, P0084 EVT Control Magnet Retarder: Description
WARNING: This page is about a different car, the 2009 Nissan Maxima. However, it is still accessible from the selected car via links, so may be relevant.
Exhaust valve timing control magnet retarder (1) controls the shut/open timing of the exhaust valve by ON/OFF pulse duty signals sent from the ECM.
The longer pulse width retards valve timing.
The shorter pulse width advances valve timing.