P0403 Code Reappears in 1999 Chrysler Voyager After 100 Miles - EGR System Diagnosis

A friend recently purchased a 1999 Chrysler Voyager with approximately 150,000 miles on its 3.3L engine. The Service Engine Soon (SES) light was illuminated, and an OBD-II scan revealed P0401 and P0403 diagnostic trouble codes. The EGR valve was removed and tested using a vacuum pump. It functioned correctly—holding open as long as vacuum was applied and releasing when vacuum was released. However, inspection of the connector to the EGR control solenoid revealed severe corrosion on one of two pins. The connector and solenoid valve were replaced. A spare used solenoid valve from a salvage yard was also installed. After disconnecting the negative battery cable to reset the codes, the vehicle operated normally for about 100 miles (roughly half a tank of fuel). Approximately five days later, the SES light returned, and scanning confirmed P040 or P0403 again. A second used solenoid valve was swapped in, and the battery cable was disconnected once more. The vehicle ran fine for another 100 miles before the issue reappeared. This raises a key question: Why does it take such a long period—over 100 miles or about one week of driving—for the SES light to return? Is there a deeper underlying issue with the EGR system, or could there be a problem with the ECU's sensor readings or wiring? Any insights into this recurring P0403 code in a 1999 Chrysler Voyager would be greatly appreciated. Thanks in advance!