P0401 Fault Diagnosis and Fix for 1996 Chevrolet Grand Prix

The P0401 diagnostic trouble code on my 1996 Chevrolet Grand Prix was caused by a severely clogged EGR port in the upper intake manifold due to carbon buildup. When I tested the EGR valve using shop air, no airflow came out from the outlet side, indicating a blockage. After clearing some of the carbon with a cleaning tool, the issue persisted during idle testing — the engine stumbled noticeably, suggesting the clog was progressing toward complete obstruction. I suspect that the upper intake manifold had been previously replaced at a junkyard, as yellow paint was visible on its back side, which may have contributed to poor sealing or long-term performance issues. I plan to verify this further with additional inspection. After completing the cleaning and ensuring proper EGR flow, the P0401 code cleared and engine performance improved significantly. The vehicle now runs smoothly without any noticeable stalling or hesitation.