  We resolved FU Ori at 29-37 GHz using the JVLA with $\sim$0$''$.07
resolution, and performed the complementary JVLA 8-10 GHz observations, the SMA
224 GHz and 272 GHz observations, and compared with archival ALMA 346 GHz
observations to obtain the SEDs. Our 8-10 GHz observations do not find evidence
for the presence of thermal radio jets, and constrain the radio jet/wind flux
to at least 90 times lower than the expected value from the previously reported
bolometric luminosity-radio luminosity correlation. The emission at $>$29 GHz
may be dominated by the two spatially unresolved sources, which are located
immediately around FU Ori and its companion FU Ori S, respectively. Their
deconvolved radii at 33 GHz are only a few au. The 8-346 GHz SEDs of FU Ori and
FU Ori S cannot be fit by constant spectral indices (over frequency). The more
sophisticated models for SEDs suggest that the $>$29 GHz emission is
contributed by a combination of free-free emission from ionized gas, and
thermal emission from optically thick and optically thin dust components. We
hypothesize that dust in the innermost parts of the disks ($\lesssim$0.1 au)
has been sublimated, and thus the disks are no more well shielded against the
ionizing photons. The estimated overall gas and dust mass based on SED
modeling, can be as high as a fraction of a solar mass, which is adequate for
developing disk gravitational instability. Our present explanation for the
observational data is that the massive inflow of gas and dust due to disk
gravitational instability or interaction with a companion/intruder, was piled
up at the few au scale due to the development of a deadzone with negligible
ionization. The piled up material subsequently triggered the thermal and the
MRI instabilities when the ionization fraction in the inner sub-au scale region
exceeded a threshold value, leading to the high protostellar accretion rate.
