P0400 Fault Code on 1997 Mitsubishi Montero FWD: Diagnosis and EGR System Monitoring Methods

P0400 Hello everyone, I own a 1997 Mitsubishi Montero (front-wheel drive, 3.5L engine — not the Sport model) that has been displaying a P0400 diagnostic trouble code, indicating an issue with exhaust gas recirculation (EGR) system flow. I've already replaced the EGR valve and cleaned the EGR ports as thoroughly as possible without disassembling the throttle body or other components. I also took the vehicle to my local Mitsubishi dealership, where they spent about three hours working on it and completed a drive cycle to prepare for emission inspection. However, upon returning for re-inspection, the system indicated that the drive cycle had not been fully completed. Approximately 20 miles later, the Check Engine light reactivated, and the P0400 code returned. I'm now wondering if there are other potential causes beyond a simple EGR valve or port issue. Could Mitsubishi use manifold absolute pressure (MAP) sensor readings to monitor EGR operation? Or is it based on exhaust temperature, oxygen sensor feedback, or throttle body/manifold pressure? I believe the MAP sensor is involved in monitoring manifold pressure, but I'm not certain about how exactly the EGR flow is validated. Any insights or troubleshooting tips would be greatly appreciated — especially for this specific model year and engine configuration. Thank you, Randy Abernathy