new valve and cleaned egr ports still the 1406 code returns any suggestions

Question

1997 gmc sonoma failed emissions high nox soon afterwards p1406 lit up my check engine light. i was expectijng it so i replaced valve and cleaned out the egr ports along with the rest of the intake manifold. 1406 reappeared almost immediately, ran a few multimeter tests i found online and it shows a low voltage reading from the pcm. supposed to be 6-9 v but it only registered 4. does this indicate a bad pcm or are there other possibillities