First, try using ratsubst instead of subst:
(C5) subst(omega23, chi * om23, chi^2 * om23^2 + 43);
(D5) chi^2*om23^2+43
(C6) ratsubst(omega23, chi * om23, chi^2 * om23^2 + 43);
(D6) omega23^2+43
If this doesn't work, you can instead try something like
subst(omega23 / chi, om23, ...)
Regards,
Barton Willis
P.S. Many thanks for the folks who helped me with Poisson and helped me
find the format package the other week; format has worked well for me and it even
contains an enhanced Poisson package. Finally,
(setq machine-mantissa-precision (float-digits 1.0d0)) works for me.