a camshaft phaser control system for reducing rotor positional instability . a phaser system includes a target wheel mounted on the phaser rotor such that during camshaft rotation wheel teeth chop a signal to generate first and second interruption signals indicative of amplitude of rotor instability . an instability monitor is used to monitor the level of instability against predetermined acceptable levels , depending upon engine operating conditions . an excessive level of instability is established by engine calibration . when measured instability exceeds a predetermined threshold level for a predetermined period , an instability diagnostic becomes alarmed . a default strategy is used to correct the excessive instability by applying a bias to the phaser control duty cycle . the system continues to monitor the level of instability , and when instability falls below the threshold limit , normal phasing operation is resumed .