EMPCD P3 (exhaust air pressure before turbine) controller feedback monitor, VGT surge clamping symptom

What it means

This code indicates that the engine control module (ECM) has detected uncontrolled oscillations or 'surging' in the exhaust pressure before the turbine. Essentially, the VGT is struggling to maintain the commanded boost/backpressure ratio, often clamping the vanes to prevent an overspeed or erratic pressure condition.