once we had a warning light which prevented us from flying an ILS app
...a
coupled ILS approach, or ILS?
If the ILS receiver is working and the restriction is on the autopilot, I can't see a reason why you can't fly it with LNAV/VNAV(or V/S), or manually.
Higher minima might be required, as the FD is not providing you with ILS guidance - comparable to raw data.
If the "warning light" (more details please), concerned the ILS receiver, then not a good idea.