C02 indicates a lack of drainage in the documented Hitachi models. First, identify whether your machine drains by pump or by gravity; the correct position of the hose depends on that setup.
The official Hitachi FAQ distinguishes between both outlet systems. That is why there is no single hose height that applies by brand.
Check the external drain path
With the appliance stopped, observe whether water is retained and whether the hose is bent, damaged, or compressed. Also check the home connection. Do not raise a gravity drain by copying a front-loading machine, or lower a pumped drain without checking its manual.
If there is a user-accessible filter, follow the safe draining steps before cleaning it: power disconnected, cold water, and containers ready. Do not open a closure while water is retained, or assume all filters have the same position.
If C02 comes back after the external drain has been checked, request a diagnosis. A pump noise can coexist with weak flow; it does not confirm proper draining or justify replacing it automatically.
Provide the reference and a description of the drain installation, especially if C02 started after a move.