  We present an analysis of archival Chandra observations of the
mixed-morphology remnant 3C400.2. We analysed spectra of different parts of the
remnant to observe if the plasma properties provide hints on the origin of the
mixed-morphology class. These remnants often show overionization, which is a
sign of rapid cooling of the thermal plasma, and super-solar abundances of
elements which is a sign of ejecta emission. Our analysis shows that the
thermal emission of 3C400.2 can be well explained by a two component
non-equilibrium ionization model, of which one component is underionized, has a
high temperature ($kT \approx 3.9$ keV) and super-solar abundances, while the
other component has a much lower temperature ($kT \approx 0.14$ keV), solar
abundances and shows signs of overionization. The temperature structure,
abundance values and density contrast between the different model components
suggest that the hot component comes from ejecta plasma, while the cooler
component has an interstellar matter origin. This seems to be the first
instance of an overionized plasma found in the outer regions of a supernova
remnant, whereas the ejecta component of the inner region is underionized. In
addition, the non-ionization equilibrium plasma component associated with the
ejecta is confined to the central, brighter parts of the remnant, whereas the
cooler component is present mostly in the outer regions. Therefore our data can
most naturally be explained by an evolutionary scenario in which the outer
parts of the remnant are cooling rapidly due to having swept up high density
ISM, while the inner parts are very hot and cooling inefficiently due to low
density of the plasma. This is also known as the relic X-ray scenario.
