Practical electrochemical devices often operate through highly coupled reaction-transport processes whose internal electrochemical states cannot be resolved from the overall cell voltage alone. This blind spot is especially consequential for membrane-electrode assemblies, where standard two-electrode measurements conflate electrode and membrane behaviours, precluding operando separation of catalytic activity, catalyst utilisation, and degradation under working conditions. Here we establish a new operando electrochemical diagnosis framework combining a polyelectrolyte-stabilised silver/ silver-bromide reference electrode and a controlled ionic connection to probe the reactive regions of working devices. This framework is proven effective across acidic, neutral and basic environments for water and CO 2 electrolysis at current densities up to 500 mA cm -2 , and enables direct and independent electrochemical characterisation of catalyst layers under practical conditions. Using this framework, we uncover a hidden transport-driven collapse in catalyst utilisation that causes more than a threefold decrease in catalyst intrinsic activity, and detect catalyst-layer degradation well before any corresponding change becomes apparent in overall cell voltages. These results indicate a pathway towards restoring electrochemical observability and accelerating the development of practical electrochemical energy systems.