All 2003-05 Civic Hybrids (ULEV and SULEV models) have two three-way catalytic converters (TWCs). If a TWC fails, it's very important to know which one has failed and to order its correct replacement part. If you replace the wrong TWC, or you replace the failed TWC with the wrong part, you could wind up with a comeback from your customer.
One TWC is near the cylinder head, and it's part of the exhaust manifold. If it fails, it causes DTC P0420 (catalyst system efficiency below threshold) to set.
The other TWC is mounted under the floor to exhaust pipe B. The under-floor TWC is either a NOX adsorptive TWC (ULEV models) or a conventional TWC (SULEV models). If the NOX adsorptive TWC fails, it causes either DTC P1420 (NOX adsorptive catalyst system efficiency below threshold) to set in '03 ULEV models or DTC P2000 (NOX adsorptive catalyst system efficiency below threshold) to set in '04-05 ULEV models. There's no DTC set if the under-floor TWC fails in a SULEV model.