97 Camaro 3800 Engine P1406 EGR Valve Pintle Position Circuit Fault After Replacement

I've been experiencing the P1406 fault code related to the EGR valve pintle position circuit on my 1997 Camaro with a 3800 engine. After replacing the EGR valve, the code returned. I confirmed that all tubes connected to the EGR are clear and free of blockages. A notable behavior I've observed is that when driving and shifting into neutral, the RPMs hover around 1500 until coming to a complete stop, after which they drop back to normal idle speed. This issue persists even after replacing the PCM due to corrosion in its connectors. I performed continuity tests on all circuits between the PCM and EGR valve and found no resistance. The reference circuit shows 5V with the key ON, and the EGR control circuit reads 12V. Despite these checks, both P1406 and erratic idle behavior remain unresolved. My emissions test is scheduled in a few weeks, so I'm seeking advice on what other potential causes or diagnostic steps to consider.