Isuzu Trooper 2002, ~100,000 miles. In good shape, runs and idles normally - idles a bit high though(~1000rpm?). I've been fighting an EGR insufficient flow code forever. Replaced EGR, cleaned input and output tubes thoroughly, cleaned TB thoroughly, removed and cleaned intake plenum THOROUGHLY. Still comes back intermittently. Replaced intake manifold gaskets. Also has several other intermittent codes that stay off for awhile and comeback occasionally then go away again: P0420 (catalyst/O2), P0446 (EVAP). Those two codes have been occasional problems for years, but we've always been able to clear them long enough to pass the smog test. Not this time though. It's the P0401 that is driving me nuts, but perhaps these are all related and I'm not aware of something that needs to be done that'll fix all three. O2 sensors are fairly new (~20k miles ago). Perhaps it's just that the cat has to be replaced? Suggestions on where to go next would be appreciated!