98 Frontier Service Engine Soon Light

[gmb...@gmail.com]

Group,

I have a 1998 Nissan Frontier FWD. I have a ECM code of 0302 (P0400)
EGR insufficient or excessive overflow detected) that I cannot
permanently clear. Between the dealer and myself, the following work
has been done (some due to other problems/codes):

MAP solenoid replaced (to clear ECM code)
entire exhaust (due to leaks/bad cat)
02 sensors replaced (to clear ECM code)
charcoal canister (to clear ECM code)
EGR (old one corroded through- I also cleaned the small and large tubes
connected to it)

It seems the SES light comes on (0302) whenever the humidity is high.
It won't pass the emissions test with this fault. Any ideas on a
permanent fix?

Thanks,
Greg